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2002 publications
 
   
 

Simulation of electronic spectra using coupled-cluster methods.
Stanton, John F.. Department of Chemistry, University of Texas,
Austin, TX, USA.

Abstracts of Papers, 223rd ACS National Meeting,
Orlando, FL, United States, April 7-11, 2002 (2002), COMP-004.
Publisher: American Chemical Society, Washington, D. C CODEN: 69CKQP
Conference; Meeting Abstract written in English. AN 2002:188405
CAPLUS (Copyright 2002 ACS)

 

Abstract

It is not widely appreciated that methods based on equation-of-motion (or
linear response) coupled-cluster theory are capable of providing a proper
description of strongly coupled potential energy surfaces, including the
phenomena assocd. with conical intersections. As a result, calcns. based
on these approaches are suitable for parametrizing vibronic Hamiltonians
that can be used in the simulation of electronic absorption and
photoelectron spectra. These approaches have been applied to the
electronic spectra of the interstellar species propadienylidene and
cyclopentadiene as well as the photoelectron and electronic absorption
spectrum of p-benzoquinone.

 
   
 

Analytic gradients for the coupled-cluster singles, doubles, and triples
(CCSDT) model. Gauss, Jurgen; Stanton, John F.. Institut fur
Physikalische Chemie, Universitat Mainz, Mainz, Germany.

Journal of Chemical Physics (2002), 116(5), 1773-1782. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 136:189594 AN 2002:75655
CAPLUS (Copyright 2002 ACS)

 

Abstract

The first implementation of analytic gradients for the coupled-cluster
singles, doubles, triples (CCSDT) model is described. The relevant theor.
expressions are given in a diagrammatic form together with the
corresponding algebraic formulas. The computational requirements of CCSDT
gradient calcns. are discussed and their applicability demonstrated by
performing benchmark calcns. for mol. geometries with large
correlation-consistent basis sets. A statistical anal. of the data reveals
that CCSDT and CCSD(T) in most cases perform equally well. The CCSDT
calcns. thus provide further evidence for the high accuracy of the CCSD(T)
approach.

 
 

 

 

 

 
 

Computational Discovery of a Novel Automerization Process for
1-Fluorocyclopropene. Dolbier, William R., Jr.; Shelton, G. Robert;
Battiste, Merle A.; Stanton, John F.; Price, David R. Department of
Chemistry, University of Florida, Gainesville, FL, USA.

 

Organic Letters (2002), 4(2), 233-235. CODEN: ORLEF7 ISSN: 1523-7060. Journal
written in English. AN 2002:14171 CAPLUS (Copyright 2002 ACS)

 

Abstract

Both DFT and CCSD and CCSD(T) computational methods indicate that
1-fluorocyclopropene can undergo an unprecedented electrocyclic
automerization process involving a full 180.degree. rotation of its
methylene group, without the formation of an intermediate carbene or
diradical.

 
 

Computational discovery of a novel automerization process for
1-fluorocyclopropene. Dolbier William R Jr; Shelton G Robert; Battiste
Merle A; Stanton John F; Price David R Department of Chemistry,
University of Florida, Gainesville, Florida 32611-7200, USA

 

.
wrd@chem.ufl.edu ORGANIC LETTERS (2002 Jan 24), 4(2), 233-5. Journal
code: 100890393. ISSN:1523-7060. Journal; Article; (JOURNAL ARTICLE)
written in English. DN 21655617 PubMed ID 11796058 AN 2002070891
MEDLINE (Copyright 2002 U.S. National Library of Medicine)

Abstract

[reaction: see text] Both DFT and CCSD and CCSD(T) computational methods
indicate that 1-fluorocyclopropene can undergo an unprecedented
"electrocyclic" automerization process involving a full 180 degrees
rotation of its methylene group, without the formation of an intermediate
carbene or diradical.

 

 

 

 
 

2001 publications

 
 

 

 

 

 
 

Coupled-cluster theory, pseudo-Jahn-Teller effects and conical
intersections. Stanton, John F.. Departments of Chemistry and
Biochemistry, Institute for Theoretical Chemistry, The University of Texas
at Austin, Austin, TX, USA.

 

Journal of Chemical Physics (2001),
115(22), 10382-10393. CODEN: JCPSA6 ISSN: 0021-9606. Journal written
in English. CAN 136:107664 AN 2001:876648 CAPLUS (Copyright 2002
ACS)

Abstract

A detailed anal. of the strengths and weaknesses of coupled-cluster and
many-body perturbation theories in treating strongly interacting potential
energy surfaces is presented. Std. coupled cluster theory is shown to
provide a qual. treatment of Herzberg-Teller coupling that is vastly
superior to that assocd. with perturbation theory. However, it also
predicts unphys. effects that will always cause it to fail in describing
the topol. of potential energy surfaces in the immediate vicinity of
conical intersections. To treat problems involving strong interstate
coupling (notably those involving radicals subject to pseudo-Jahn-Teller
effects), methods based on equation-of-motion (linear response)
coupled-cluster theory appear to be considerably more suitable. In
particular, they provide a description of intersecting surfaces that is
qual. correct in all respects. It is also shown that there is no reason to
believe that the noniterative inclusion of triple excitation contributions
to the correlation energy should provide for any systematic improvement in
describing this class of phenomena.

 
 

 

 

 

 
 

Microwave Spectra and Molecular Structures of (Z)-Pent-2-en-4-ynenitrile
and Maleonitrile. Halter, Robert J.; Fimmen, Ryan L.; McMahon, Robert
J.; Peebles, Sean A.; Kuczkowski, Robert L.; Stanton, John F..
Department of Chemistry, University of Wisconsin, Madison, WI, USA.


Journal of the American Chemical Society (2001), 123(49), 12353-12363.
CODEN: JACSAT ISSN: 0002-7863. Journal written in English. CAN
136:85572 AN 2001:818811 CAPLUS (Copyright 2002 ACS)

Abstract

Accurate equil. structures have been detd. for (Z)-pent-2-en-4-ynenitrile
(8) and maleonitrile (9) by combining microwave spectroscopy data and ab
initio quantum chem. calcns. The microwave spectra of 10 isotopomers of 8
and 5 isotopomers of 9 were obtained using a pulsed nozzle Fourier
transform microwave spectrometer. The ground-state rotational consts. were
adjusted for vibration-rotation interaction effects calcd. from force
fields obtained from ab initio calcns. The resultant equil. rotational
consts. were used to det. structures that are in very good agreement with
those obtained from high-level ab initio calcns. (CCSD(T)/cc-pVTZ). The
geometric parameters in 8 and 9 are very similar; they also do not differ
significantly from the all-carbon analog, (Z)-hex-3-ene-1,5-diyne (7), the
parent mol. for the Bergman cyclization. A small deviation from linearity
about the alkyne and cyano linkages is obsd. for 7-9 and several related
species where accurate equil. parameters are available. The data on 7-9
should be of interest to radioastronomy and may provide insights on the
formation and interstellar chem. of unsatd. species such as the
cyanopolyynes. Safety: authors caution that several of the synthetic
intermediates for prepn. of 8 are severe vesicants and care should be taken
when handling them.

 
 

 

 

 

 
 

A comparison of excited state properties for iterative approximate triples
linear response coupled cluster methods. Sattelmeyer, Kurt W.; Stanton,
John F.; Olsen, Jeppe; Gauss, Jurgen. Departments of Chemistry and
Biochemistry, Institute for Theoretical Chemistry, The University of Texas
at Austin, Austin, TX, USA.

 

Chemical Physics Letters (2001),
347(4,5,6), 499-504. CODEN: CHPLBC ISSN: 0009-2614. Journal written in
English. CAN 136:25509 AN 2001:802507 CAPLUS (Copyright 2002
ACS)

Abstract

A computational study of the potential energy curves of the 1.PI. state of
BH, 1.PI. state of CH+, 1.SIGMA.u and 1.PI.u states of C2, 1.PI. state of
CO, and 1.PI.g and 1.SIGMA.-u states of N2 is carried out with the CC3 and
CCSDT-3 corrections to EOMEE-CCSD. Good agreement in structure,
vibrational frequencies, and excitation energies of these iterative
triples-cor. methods with respect to expt. is found for most of these
examples. However, deficiencies in the approx. treatment of triples is
evident for BH and CH+.

 
 

 

 

 

 
 

On the photoelectron spectrum of p-benzoquinone. Stanton, John F.;
Sattelmeyer, Kurt W.; Gauss, Jurgen; Allan, Michael; Skalicky, Tomas;
Bally, Thomas. Departments of Chemistry and Biochemistry, Institute for
Theoretical Chemistry, The University of Texas at Austin, Austin, TX,
USA.

 

Journal of Chemical Physics (2001), 115(1), 1-4. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 135:217964 AN
2001:458227 CAPLUS (Copyright 2002 ACS)

Abstract

A high-resoln. photoelectron spectrum of p-benzoquinone in the low energy
(9.5-11.5 eV) region is reported and analyzed with the aid of simulations
based on high-level ab initio calcns. The results generally support the
notion that the two prominent spectral features in this region are each due
to a pair of final ion states. The lower energy feature beginning near 10
eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV
comes from .pi. electron removal. Contrary to previous interpretations of
the spectrum, however, the results of this study indicate that the two .pi.
states are nearly degenerate, with the strongest peak in the photoelectron
spectrum representing a convolution of the corresponding pair of 0-0
ionizations.

 
 

 

 

 

 
 

Problematic p-benzyne: Orbital instabilities, biradical character, and
broken symmetry. Crawford, T. Daniel; Kraka, Elfi; Stanton, John F.;
Cremer, Dieter. Department of Chemistry, Virginia Tech, Blacksburg,
VA, USA

 

. Journal of Chemical Physics (2001), 114(24), 10638-10650.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
135:226574 AN 2001:439357 CAPLUS (Copyright 2002 ACS)

Abstract

The equil. geometry, harmonic vibrational frequencies, and IR transition
intensities of p-benzyne were calcd. at the MBPT(2), SDQ-MBPT(4), CCSD, and
CCSD(T) levels of theory using different ref. wave functions obtained from
restricted and UHF (RHF and UHF), restricted Brueckner (RB) orbital, and
Generalized Valence Bond (GVB) theory. RHF erroneously describes p-benzyne
as a closed-shell singlet rather than a singlet biradical, which leads to
orbital near-instabilities in connection with the mixing of orbital pairs
b1u-ag(HOMO-LUMO), b2g-ag(HOMO-1-LUMO), and b1g-ag(HOMO-2-LUMO).
Vibrational modes of the corresponding symmetries cause method-dependent
anomalous increases (unreasonable force consts. and IR intensities) or
decreases in the energy (breaking of the D2h symmetry of the mol. framework
of p-benzyne). This basic failure of the RHF starting function is reduced
by adding dynamic electron correlation. However RHF-MBPT(2),
RHF-SDQ-MBPT(4), RHF-CCSD, RB-CCD, and RHF-CCSD(T) descriptions of
p-benzyne are still unreliable as best documented by the properties of the
b1u-, b2g-, and b1g-sym. vibrational modes. The first reliable
spin-restricted description is provided when using Brueckner orbitals at
the RB-CCD(T) level. GVB leads to exaggerated biradical character that is
reduced at the GVB-MP2 level of theory. The best results are obtained with
a UHF ref. wave function, provided a sufficient account of dynamic electron
correlation is included. At the UHF-CCSD level, the triplet contaminant is
completely annihilated. UHF-CCSD(T) gives a reliable account of the IR
spectrum apart from a CCH bending vibrational mode, which is still in
disagreement with expt.

 
 

 

 

 

 
 

The equilibrium structure of the ammonium radical Rydberg ground state.
Sattelmeyer, Kurt W.; Schaefer, Henry F., III; Stanton, John F.. Center
for Computational Quantum Chemistry, University of Georgia, Athens, GA,
USA.

 

Journal of Chemical Physics (2001), 114(22), 9863-9865. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 135:157923
AN 2001:391262 CAPLUS (Copyright 2002 ACS)

Abstract

The equil. structure of the ammonium radical is detd. by an extrapolation
procedure of large [up to CCSD(T)/aug-cc-pV5Z] coupled cluster calcns. and
anal. of exptl. rotational consts. cor. for effects of rotation-vibration
interaction. These approaches yield re values of 1.0367 and 1.0363 .ANG.,
resp. The small difference serves as a valuable internal consistency check
and suggests that re in NH4 lies in the range 1.0365 .+-. 0.0005 .ANG..

 
 

 

 

 

 
 

The accurate determination of molecular equilibrium structures. Bak,
Keld L.; Gauss, Jurgen; Jorgensen, Poul; Olsen, Jeppe; Helgaker, Trygve;
Stanton, John F.. UNI-C, Aarhus, Den.

 

Journal of Chemical Physics
(2001), 114(15), 6548-6556. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 134:316282 AN 2001:249364 CAPLUS
(Copyright 2002 ACS)

Abstract

Equil. structures of 19 mols. were detd. by using least-squares fits
involving rotational consts. from expt. and vibrational corrections from
high-level electronic-structure calcns. Equil. structures obtained by this
procedure have a uniformly high quality. Indeed, the accuracy of the
results reported here likely surpasses that reported in most exptl. detns.
In addn., the accuracy of equil. structures obtained by energy minimization
was calibrated for the following std. models of ab initio theory:
Hartree-Fock, MP2, CCSD, and CCSD(T). In accordance with previous
observations, CCSD(T) is significantly more accurate than the other models;
the mean and max. abs. errors for bond distances of the 19 mols. are 0.09
and 0.59 pm, resp., in CCSD(T)/cc-pCVQZ calcns. The max. error is obtained
for ROO in H2O2 and is so large compared with the mean abs. error that an
exptl. reinvestigation of this mol. is warranted.

 
 

 

 

 

 
 

Simulation of vibronic spectra using coupled-cluster excited state methods.
Stanton, John F.; Sattelmeyer, Kurt W. Departments of Chemistry and
Biochemistry, University of Texas at Austin, Austin, TX, USA.

 

Abstr.
Pap. - Am. Chem. Soc. (2001), 221st PHYS-170. CODEN: ACSRAL ISSN:
0065-7727. Journal; Meeting Abstract written in English. AN
2001:203458 CAPLUS (Copyright 2002 ACS)

Abstract

It is not widely appreciated that methods based on equation-of-motion (or
linear response) coupled-cluster theory are capable of providing a proper
description of strongly coupled potential energy surfaces, including the
phenomena assocd. with conical intersections. As a result, calcns. based
on these approaches are suitable for parametrizing vibronic Hamiltonians
that can be used in the simulation of electronic absorption and
photoelectron spectra. These approaches have been applied to the
electronic spectra of the interstellar species propadienylidene and
cyclopentadiene as well as the photoelectron and electronic absorption
spectrum of p-benzoquinone.

 
 

 

 

 

 
 

Equilibrium geometries of cyclic SiC3 isomers. Stanton, John F.; Gauss,
Jurgen; Christiansen, Ove. Institute for Theoretical Chemistry,
Departments of Chemistry and Biochemistry, The University of Texas at
Austin, Austin, TX, USA.

 

Journal of Chemical Physics (2001),
114(7), 2993-2995. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 134:316388 AN 2001:83911 CAPLUS (Copyright 2002ACS)

Abstract

Equil. geometries of two isomers of cyclic SiC3 are detd. by both large
scale ab initio calcn. and a procedure involving the use of exptl. data
from microwave spectroscopy and calcd. harmonic and cubic force fields.
Internuclear distances corresponding to structures obtained by both
procedures agree to better than 0.003 .ANG. in all cases, allowing for
precise recommendations of equil. structures. Rather large residual
inertial defects obtained from moments of inertia based on rotational
consts. adjusted for effects of vibration-rotation interaction were found
to be reduced significantly by inclusion of electronic contributions as
estd. from calcns. of rotational g-tensors.

 

 

 

 
2000 publications
 

 

 

 


Electronic spectrum of propadienylidene (H2C=C=C:) and its relevance to the
diffuse interstellar bands. Hodges, Jonathan A.; McMahon, Robert J.;
Stanton, John F.; Sattelmeyer, Kurt W. Department of Chemistry,
University of Wisconsin, Madison, WI, USA.

 

Astrophysical Journal
(2000), 544(2, Pt. 1), 838-842. CODEN: ASJOAB ISSN: 0004-637X. Journal
written in English. CAN 134:229059 AN 2001:35342 CAPLUS
(Copyright 2002 ACS)

Abstract

The electronic spectrum of singlet propadienylidene (H2C=C=C:) matrix
isolated in Ne at 6 K was measured. Propadienylidene is the only member of
the homologous series of neutral C chain mols. H2C(=C)n to be obsd. by
electronic spectroscopy. The spectrum displays three electronic
transitions: two in the visible (A 1A2 .rarw. X 1A1 and B 1B1 .rarw. X 1A1)
and one in the UV (C 1A1 .rarw. X 1A1). The longest wavelength transition
(A 1A2 .rarw. X 1A1) is symmetry forbidden but is weakly seen in the
spectrum via vibronic coupling with the B 1B1 state. The other two
transitions exhibit well-resolved vibrational progressions. The high d. of
absorptions in the visible region confirms earlier suggestions that the
H2C(=C)n family of cumulene carbenes merits careful consideration as
potential carriers of diffuse interstellar bands.

 
 

 

 

 

 
 

Barrier to Methyl Internal Rotation of Cis- and Trans-2-Methylvinoxy
Radicals in the .lambda.lX(2A'') and .lambda.lB(.lambda.l2A'') States:
Experiment and Theory. Williams, Sarah; Harding, Lawrence B.; Stanton,
John F.; Weisshaar, James C. Department of Chemistry, University of
Wisconsin-Madison, Madison, WI, USA.

 

J. Phys. Chem. A (2000),
104(44), 9906-9913. CODEN: JPCAFH ISSN: 1089-5639. Journal written in
English. CAN 133:334976 AN 2000:720492 CAPLUS (Copyright 2002
ACS)

Abstract

The jet-cooled laser induced fluorescence spectrum of the ~B .rarw. ~X
electronic transition of 2-methylvinoxy radical is assigned as a
superposition of contributions from noninteracting cis and trans isomers.
The spectrum of the cis isomer is identified by comparison with ab initio
electronic structure calcns.; both theory and expt. clearly indicate that
the Me conformation changes from the ~X state to the ~B state. Fits of
both hot and cold bands to a one-dimensional torsional model yield Me rotor
barrier magnitudes of 270 .+-. 20 cm-1 in the ~X state and 200 .+-. 20 cm-1
in the ~B state. The ab initio calcns. show that in the ground state the
preferred conformation places one Me CH bond in the plane of the mol. frame
cis to the vicinal CC bond. Assignment of the spectrum of
trans-2-methylvinoxy is more tentative because no resolved hot bands are
available to corroborate the model. Our best est. for the ~B state barrier
magnitude is 60 .+-. 15 cm-1. Multireference CI calcns. and coupled
cluster calcns. are reasonably successful in obtaining Me torsional
barriers in agreement with expt., although high accuracy is elusive for the
~B state of both cis and trans isomers. By comparison with simpler cases,
we infer that the .pi. radical character of the ~B state strongly
influences the Me torsional barrier.

 
 

 

 

 

 
 

Computational Studies of C6H2 Isomers. Sattelmeyer, Kurt W.; Stanton,
John F.. Institute for Theoretical Chemistry Departments of Chemistry
and Biochemistry, University of Texas, Austin, TX, USA.

 

J. Am. Chem.
Soc. (2000), 122(34), 8220-8227. CODEN: JACSAT ISSN: 0002-7863.
Journal written in English. CAN 133:281334 AN 2000:567749 CAPLUS
(Copyright 2002 ACS)

Abstract

High-level ab initio methods are used to characterize nine isomers of C6H2.
These include the thermodynamically favored triacetylene form and
hexapentaenylidene, which is known to exist in the interstellar medium.
Also included in the survey are three cyclic forms of tetradehydrobenzene
and four addnl. structures that have not been investigated previously.
Structures, dipole moments, parameters of rotational Hamiltonians,
vibrational frequencies, intensities, and 13C isotopic shifts are
documented for all forms to assist efforts aimed at detecting these species
in the lab. Perhaps the best candidate for detection in space is
ethynylbutatriehylidene, which has a substantial dipole moment and a
thermodn. stability comparable to that of hexapentaenylidene. Notably, the
sym. disubstituted vinylidene isomer has a small and perhaps vanishing
barrier to rearrangement.

 
 

 

 

 

 
 

Barrier to Methyl Internal Rotation of 1-Methylvinoxy Radical in the
.lambda.1X(2A'') and .lambda.1B(2A'') States: Experiment and Theory.
Williams, Sarah; Harding, Lawrence B.; Stanton, John F.; Weisshaar, James
C. Department of Chemistry, University of Wisconsin-Madison, Madison,
WI, USA.

 

Journal of Physical Chemistry A (2000), 104(45),
10131-10138. CODEN: JPCAFH ISSN: 1089-5639. Journal written in English.
CAN 133:281374 AN 2000:410610 CAPLUS (Copyright 2002 ACS)

Abstract

The jet-cooled laser induced fluorescence spectrum of the ~B .rarw. ~X
electronic transition of the 1-methylvinoxy radical is assigned, including
both hot and cold bands. The barrier to Me internal rotation in both ~X
and ~B states is detd. by fitting pure torsional transitions to a
1-dimensional hindered-rotor model. The resulting 3-fold torsional barrier
parameters are V3' = -740 .+-. 30 cm-1 for the ~B state (min.-energy
conformation with one Me CH bond cis to the frame CO bond) and V3'' = +130
.+-. 30 cm-1 for the ~X state (Me CH bond trans to CO). The intensity
pattern clearly indicates a change in the preferred Me conformation upon
excitation, while ab initio calcns. provide the abs. conformations in each
state. A variety of ab initio methods including CASSCF, multireference CI,
and coupled-cluster techniques were applied to both the ~X and the ~B
states of 1-methylvinoxy. Only the largest coupled-cluster calcns. yield a
~B-state barrier in good quant. agreement with expt. In unsubstituted
vinoxy, a ~B-state geometry adjusted earlier to fit exptl. rotational
consts. is evidently in error.

 
 

 

 

 

 
 

Analytic second derivatives in high-order many-body perturbation and
coupled-cluster theories: computational considerations and applications.
Stanton, John F.; Gauss, Jurgen. Institute for Theoretical Chemistry,
Departments of Chemistry and Biochemistry, The University of Texas at
Austin, Austin, TX, USA.

 

International Reviews in Physical Chemistry
(2000), 19(1), 61-95. CODEN: IRPCDL ISSN: 0144-235X. Journal; General
Review written in English. CAN 133:22524 AN 2000:365177 CAPLUS
(Copyright 2002 ACS)

Abstract

A review with 130 refs. The history of analytic first- and second-deriv.
methods in quantum chem. is discussed, with special emphasis given to
approaches that are assocd. with electron correlation treatments based on
many-body perturbation theory (MBPT) and the coupled-cluster (CC) approxn.
The computational requirements of recently developed anal. second deriv.
methods for high-order MBPT and CC methods are discussed in detail and
compared with those assocd. with finite-difference procedures.
Applications of these techniques to the calcn. of anharmonic force fields
used to deduce equil. geometries and fundamental vibrational frequencies
for polyat. mols. are reviewed.

 
 

 

 

 

 
 

Analytic first and second derivatives for the CCSDT-n (n = 1-3) models: a
first step towards the efficient calculation of CCSDT properties.
Gauss, Jurgen; Stanton, John F.. Institut fur Physikalische Chemie,
Universitat Mainz, Mainz, Germany.

 

Physical Chemistry Chemical Physics
(2000), 2(10), 2047-2060. CODEN: PPCPFQ ISSN: 1463-9076. Journal
written in English. CAN 133:49053 AN 2000:303804 CAPLUS
(Copyright 2002 ACS)

Abstract

Analytic first and second derivs. of the energy are implemented for
closed-shell systems described by the CCSDT-n (n=1, 1b, 2 and 3) and CC3
electron correlation models. A detailed discussion of the computational
requirements of these calcns. is given, along with diagrammatic formulas
for all relevant quantities. The method is applied to calc. the nuclear
magnetic shielding of H2O, CO and N2O and the structure and properties of
propadienylidene.

 
 

 

 

 

 
 

Some surprising failures of Brueckner coupled cluster theory. Crawford,
T. Daniel; Stanton, John F.. Departments of Chemistry and Biochemistry,
Institute for Theoretical Chemistry, The University of Texas, Austin,
TX, USA.

 

Journal of Chemical Physics (2000), 112(18), 7873-7879.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
133:64174 AN 2000:287098 CAPLUS (Copyright 2002 ACS)

Abstract

Brueckner coupled cluster (B-CC) methods have seen a considerable rise in
popularity over the last decade thanks, in part, to their apparent
propensity for avoiding artifactual symmetry-breaking problems that
sometimes plague Hartree-Fock-based approaches. Recent B-CC applications
to problematic systems such as the tetraoxygen cation have provided
encouraging examples of the success of this theory. In the present work,
we examine the performance of the Brueckner technique for a no. of other
well-known symmetry-breaking problems, including the formyloxyl radical,
the first excited state of NO2 and the nitrate radical. In these cases,
B-CC methods are found to fail dramatically, predicting broken-symmetry
equil. geometries in conflict with exptl. and/or higher-level theor.
results. A framework is developed which indicates that these errors can be
attributed to artificially exaggerated second-order Jahn-Teller
interactions with nearby electronic states. Hence, in spite of their
initial successes, Brueckner methods cannot be considered a panacea for
symmetry-breaking problems.

 
 

 

 

 

 
 

Reviews in Computational Chemistry, Volume 13. Edited by Kenny B. Lipkowitz
and Donald B. Boyd. Stanton, John F.. Department of Chemistry,
University of Texas, Austin, TX, USA.

 

J. Comput. Chem. (2000),
21(6), 505-507. CODEN: JCCHDD ISSN: 0192-8651. Journal; Book Review
written in English. AN 2000:223401 CAPLUS (Copyright 2002 ACS)

 
 

 

 

 

 
 

Pauling's Legacy: Modern Modelling of the Chemical Bond. Theoretical &
Computational Chemistry. Volume 6. Edited by Z. B. Maksic (Rudjer Boskovic
Institute, Zagreb) and W. J. Orville-Thomas (Aberystwyth, Wales). Elsevier:
Amsterdam. 1999. 782 pp. $360.50. ISBN 0-444-82508-8. Stanton, John F..
University of Texas at Austin, USA

 

J. Am. Chem. Soc. (2000), 122(19),
4846. CODEN: JACSAT ISSN: 0002-7863. Journal; Book Review written in
English. AN 2000:196480 CAPLUS (Copyright 2002 ACS)

 
 

 

 

 

 
 


The Equilibrium Structure of Benzene. Gauss, Juergen; Stanton, John F..
Institut fuer Physikalische Chemie, Universitaet Mainz, Mainz,
Germany.

 

J. Phys. Chem. A (2000), 104(13), 2865-2868. CODEN: JPCAFH
ISSN: 1089-5639. Journal written in English. CAN 132:278773 AN
2000:167586 CAPLUS (Copyright 2002 ACS)

Abstract

The re structure of benzene is revised on the basis of high-level quantum
chem. calcns. at the CCSD(T)/cc-pVQZ level as well a reanal. of the exptl.
rotational consts. using computed vibrational corrections. A least-squares
fit to empirically detd. Be consts. yields re(CC) = 1.3914 .+-. 0.0010
.ANG. and re(CH) = 1.0802 .+-. 0.0020 .ANG.; the latter distance is
significantly shorter than the best previous est. based on exptl. data.
Comparison of computed rg and rz distances with expt. as well as
considerations of bond lengthening due to anharmonicity are consistent with
the estd. re distance, indicating that the recommended structural
parameters are very accurate.

 
 

 

 

 

 
 

Equilibrium structure of LiCCH. Gauss, Jurgen; Stanton, John F..
Institut fur Physikalische Chemie, Universitat Mainz, Mainz, Germany.

 


Int. J. Quantum Chem. (2000), 77(1), 305-310. CODEN: IJQCB2 ISSN:
0020-7608. Journal written in English. CAN 132:227707 AN
2000:101105 CAPLUS (Copyright 2002 ACS)

Abstract

A coupled-cluster study of LiCCH is presented to provide state-of-the-art
calcns. for the equil. geometry of LiCCH and to supply vibration-rotation
interaction consts., thus making possible the detn. of an re structure
based on rotational consts. from a recent microwave investigation of
various isotopomers of LiCCH. The power of this combined exptl. and theor.
approach to accurate equil. geometries is emphasized.

 
 

 

 

 

 
 

The re Structure of Cyclopropane. Gauss, Juergen; Cremer, Dieter;
Stanton, John F.. Institut fuer Physikalische Chemie, Universitaet
Mainz, Mainz, Germany.

 

J. Phys. Chem. A (2000), 104(6), 1319-1324.
CODEN: JPCAFH ISSN: 1089-5639. Journal written in English. CAN
132:207492 AN 2000:59605 CAPLUS (Copyright 2002 ACS)

Abstract

A long-standing controversy regarding the re structure of cyclopropane is
resolved by performing high-level quantum chem. calcns. and analyzing the
exptl. rotational consts. for C3H6 and C3H4D2 augmented by calcd.
vibrational corrections. For the latter, a least-squares fit yields the
following set of parameters: re (CC) = 1.5030(10) .ANG., re(CH) =
1.0786(10) .ANG., and .alpha.e(HCH) = 114.97(10).degree., which compare
favorably with both the pure computational result obtained at the
CCSD(T)/cc-pVQZ level as well as an earlier est. of the re structure of
cyclopropane based on anal. of gas-phase electron diffraction data. Our
results are in rather poor agreement with a structure based on a previous
anal. of the rotational consts. that used empirically estd. vibrational
corrections.

 
 

 

 

 

 
 

Equilibrium Structure of cis-Hex-3-ene-1,5-diyne and Relevance to the
Bergman Cyclization. McMahon, Robert J.; Halter, Robert J.; Fimmen,
Ryan L.; Wilson, Robb J.; Peebles, Sean A.; Kuczkowski, Robert L.; Stanton,
John F.. Department of Chemistry, University of Wisconsin, Madison,
WI, USA.

 

J. Am. Chem. Soc. (2000), 122(5), 939-949. CODEN: JACSAT
ISSN: 0002-7863. Journal written in English. CAN 132:193985 AN
2000:44094 CAPLUS (Copyright 2002 ACS)

Abstract

An accurate gas-phase structure detn. of cis-hex-3-ene-1,5-diyne (1), the
parent mol. for the Bergman cyclization reaction, has been achieved through
a combination of Fourier transform microwave spectroscopy and computational
quantum chem. The microwave spectra of seven isotopes of enediyne 1 have
been obsd. using a pulsed-nozzle Fourier transform spectrometer. The
ground-state rotational consts. were cor. for vibrational effects and the
equil. structure (Re) was deduced. The equil. structure displays excellent
agreement with that obtained from high-level ab initio calcns.
(CCSD(T)/cc-pVTZ). Both approaches confirm that the mol. has C2v symmetry
with slight deviations of the alkyne units (C-C.tplbond.C-H) from
linearity. These groups distort away from each other in contra-distinction
to their distortion during the Bergman cyclization reaction. The C1-C6
distance (4.32 .ANG.), which has been viewed as a crit. parameter governing
reactivity in the Bergman cyclization, is notably longer than an earlier,
widely publicized value (4.12 .ANG.). An important role of the macrocyclic
ring in enediyne antibiotics is to overcome an intrinsic distortion of the
enediyne moiety in the direction opposite that of the reaction coordinate
for Bergman cyclization.

 

 

 
1999 publications
 

 

 

 

The electronic spectrum of pyrrole. Christiansen, Ove; Gauss, Jurgen;Stanton, John F.; Jorgensen, Poul. Institut fur Physikalische Chemie,
Universitat Mainz, Mainz, Germany.

 

J. Chem. Phys. (1999), 111(2),
525-537. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
(Correction of: CAN 131:175263 AN 1999:400260) CAN 132:55084 AN
2000:28532 CAPLUS (Copyright 2002 ACS)

Abstract

The electronic spectrum of pyrrole has been investigated by performing
calcns. using a hierarchy of coupled-cluster models consisting of CCS, CC2,
CCSD, and CC3. Basis-set effects have been investigated by carrying out
calcns. using correlation-consistent basis sets augmented with functions
esp. designed for the description of Rydberg states. Oscillator strengths,
excited state dipole moments, and second moments of the electronic charge
distributions have been used to characterize the electronic transitions and
final states. Structures and vibrational frequencies have been calcd. for
a few selected states, and the importance of distinguishing between
vertical and adiabatic transitions for accurate comparison with expt. has
been emphasized. The exptl. spectrum has been scrutinized in the relevant
energy region, and the accuracy of recent calcns. [CASPT2, MRMP, ADC(2)]
has been critically reexamd.

 
 

 

 

 

 
 

Conformations of [10]Annulene: More Bad News for Density Functional Theory
and Second-Order Perturbation Theory. King, Rollin A.; Crawford,T.
Daniel; Stanton, John F.; Schaefer, Henry F. ,III. Center for
Computational Quantum Chemistry Department of Chemistry, The University of
Georgia, Athens, GA, USA.

 

J. Am. Chem. Soc. (1999), 121(46),
10788-10793. CODEN: JACSAT ISSN: 0002-7863. Journal written in English.
CAN 132:35368 AN 1999:705505 CAPLUS (Copyright 2002 ACS)

Abstract

The mol. structures and relative energies of several of the lowest-energy
conformations of [10]annulene (C10H10) have been investigated using the
Hartree-Fock (HF) method, d. functional theory (DFT), second-order
Moller-Plesset perturbation theory (MP2), and (for the first time) coupled
cluster singles and doubles with a perturbative inclusion of connected
triple excitations [CCSD(T)]. For some years the HF method has been known
to incorrectly favor bond-length-alternating structures for [10]annulene,
and std. forms of DFT are now seen to incorrectly favor arom. structures.
For the naphthalene-like conformation, the B3LYP method is shown to require
a large basis set before the geometry becomes properly bond-localized,
i.e., similar to the predictions of CCSD(T) using even a modest basis set.
With a basis set of 170 functions, B3LYP and BP86 predict that the arom.
heart-shaped conformation is 9.11 and 12.11 kcal mol-1, resp., lower than
the bond-alternating twist form, while with the same basis set CCSD(T)
places the heart-shaped conformation 6.29 kcal mol-1 above the twist.
Further large-scale CCSD(T) computations involving 340 basis functions
predict that the twist conformation is lowest in energy, and the
naphthalene-like and heart-shaped conformations lie higher than the twist
by 1.40 and 4.24 kcal mol-1, resp. Implications of the computed structures
and energetics for the interpretation of previous expts. are discussed.

 
 

 

 

 

 
 

A simple scheme for the direct calculation of ionization potentials with
coupled-cluster theory that exploits established excitation energy methods.
Stanton, John F.; Gauss, Jurgen. Departments of Chemistry and
Biochemistry, Institute for Theoretical Chemistry, The University of Texas
at Austin, Austin, TX, USA.

 

J. Chem. Phys. (1999), 111(19),
8785-8788. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 132:27040 AN 1999:696554 CAPLUS (Copyright 2002 ACS)

Abstract

Vertical ionization potentials can be obtained from existing computer
programs for the high-level treatment of excited states by simply including
a continuum orbital in the basis set. Exploiting this feature easily
allows final state energies for ionized states to be calcd. at several
previously untested levels of theory that go beyond the equation-of-motion
coupled-cluster singles and doubles model. Values obtained for N2, CO, and
F2 with the most theor. complete approxns. studied here (those based on the
CCSDT-3 and CC3 parametrizations of the neutral ground state) are in
excellent agreement with expt. when a large basis set is used.

 
 

 

 

 

 
 

A refined estimate of the bond length of methane. Stanton, John F..
Institute for Theoretical Chemistry, Departments of Chemistry and
Biochemistry, The University of Texas at Austin, Austin, TX, USA.

 

Mol. Phys. (1999), 97(7), 841-845. CODEN: MOPHAM ISSN: 0026-8976.
Journal written in English. CAN 132:12062 AN 1999:684740 CAPLUS
(Copyright 2002 ACS)

Abstract

The equil. structure of methane is detd. by two different procedures. The
first is based on high level ab initio calcns. using the CCSD(T) treatment
of electron and basis sets contg. up to 365 contracted Gaussian functions.
Using an extrapolation procedure to account for residual inadequacies of
the basis set and an est. of higher order correlation effects, an equil.
internuclear sepn. of 1.08595 .+-. 0.0003 .ANG. is obtained. This value is
in almost perfect agreement with that obtained by correcting measuring
ground-state rotational consts. of methane isotopomers for vibrational
effects and using the resulting empirical equil. rotational consts. to
infer the equil. structure.

 
 

 

 

 

 
 

Application of an equation-of-motion coupled cluster method including
higher-order corrections to potential energy surfaces of radicals.
Saeh, Jamal C.; Stanton, John F.. Departments of Chemistry and
Biochemistry, Institute for Theoretical Chemistry, The University of Texas
at Austin, Austin, TX, USA.

 

J. Chem. Phys. (1999), 111(18),
8275-8285. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 132:40671 AN 1999:677372 CAPLUS (Copyright 2002 ACS)

Abstract

A method proposed some time ago to correct for residual correlation effects
within the framework of equation-of-motion coupled-cluster theory for
ionized states in the singles and doubles approxn. (EOMIP-CCSD) is
discussed, and a minor modification is suggested that provides a more
balanced theor. treatment. The resulting approach is used to calc.
properties of diat. radicals, where comparison with expt. and other
high-level calcns. is straightforward. In addn., two electronic states of
NO2 and the ground state of the cyclic C3H mol. are studied, cases in which
symmetry-breaking phenomena play an important role. Results indicate that
EOMIP-CCSD* generally provides a systematic improvement relative to the
uncorrected EOMIP-CCSD approach for predicting properties of radicals.

 
 

 

 

 

 
 

Rotational spectrum and theoretical structure of the carbene HC4N.
McCarthy, M. C.; Apponi, A. J.; Gordon, V. D.; Gottlieb, C. A.; Thaddeus,
P.; Daniel Crawford, T.; Stanton, John F.. Harvard-Smithsonian Center
for Astrophysics, Cambridge, MA, USA.

 

J. Chem. Phys. (1999),
111(15), 6750-6754. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 132:16656 AN 1999:627060 CAPLUS (Copyright 2002
ACS)

Abstract

Following a high-level coupled cluster calcn., the rotational spectrum of
the bent HC4N singlet carbene was detected in a supersonic mol. beam by
Fourier transform microwave spectroscopy. The three rotational consts.,
the leading centrifugal distortion consts., and two N hyperfine coupling
consts. high accuracy. The rotational consts. agree with those calcd. ab
initio to better than 0.5%. Like the isoelectronic C5H2 carbene of similar
structure, HC4N has fairly large centrifugal distortion and a large
inertial defect. The calcd. dipole moment of HC4N is 2.95 D.

 
 

 

 

 

 
 

The electronic spectrum of pyrrole. Christiansen, Ove; Gauss, Jurgen;
Stanton, John F.; Jo/rgensen, Poul. Institut fur Physikalische Chemie,
Universitat Mainz, Mainz, Germany.

 

J. Chem. Phys. (1999), 111(2),
525-537. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 131:175263 AN 1999:400260 CAPLUS (Copyright 2002 ACS)

Abstract

The electronic spectrum of pyrrole has been investigated by performing
calcns. using a hierarchy of coupled-cluster models consisting of CCS, CC2,
CCSD, and CC3. Basis-set effects have been investigated by carrying out
calcns. using correlation-consistent basis sets augmented with functions
esp. designed for the description of Rydberg states. Oscillator strengths,
excited state dipole moments, and second moments of the electronic charge
distributions have been used to characterize the electronic transitions and
final states. Structures and vibrational frequencies have been calcd. for
a few selected states, and the importance of distinguishing between
vertical and adiabatic transitions for accurate comparison with expt. has
been emphasized. The exptl. spectrum has been scrutinized in the relevant
energy region, and the accuracy of recent calcns. [CASPT2, MRMP, ADC(2)]
has been critically reexamd.

 
 

 

 

 

 
 

Frequency-dependent polarizabilities and first hyperpolarizabilities of CO
and H2O from coupled cluster calculations. Christiansen, Ove; Gauss,
Jurgen; Stanton, John F.. Institut fur Physikalische Chemie,
Universitat Mainz, Mainz, Germany.

 

Chem. Phys. Lett. (1999),
305(1,2), 147-155. CODEN: CHPLBC ISSN: 0009-2614. Journal written in
English. CAN 131:52623 AN 1999:342986 CAPLUS (Copyright 2002
ACS)

Abstract

Frequency-dependent electronic polarizabilities and 1st
hyperpolarizabilities for CO and H2O were studied in coupled-cluster
response theory calcns. Triple excitation effects were considered by the
CC3 model which has recently been implemented for frequency-dependent
polarizabilities and 1st hyperpolarizabilities. The final ests. for
polarizabilities and 1st hyperpolarizabilities are in good agreement with
exptl. results.

 
 

 

 


 
 

Vibrational structure in the vinylidene anion photoelectron spectrum:
Closing the gap between theory and experiment. Stanton, John F.; Gauss,
Jurgen. Departments of Chemistry and Biochemistry, Institute for
Theoretical Chemistry, The University of Texas at Austin, Austin, TX,
USA.

 

J. Chem. Phys. (1999), 110(12), 6079-6080. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 130:303563 AN
1999:175012 CAPLUS (Copyright 2002 ACS)

Abstract

Photoelectron spectra of the vinylidene anion exhibit rich vibrational
structure, including several transitions of the neutral that lie well above
the barrier for rearrangement to acetylene. Most of these features were
assigned, and are largely consistent with vibrational energy levels estd.
by ab initio calcns. Notable exceptions include the 602 and 611 vibronic
transitions which involve the CH2 rocking coordinate that ultimately leads
to isomerization. Very poor agreement between theory and expt. was
attributed to a neglect of vibrational anharmonicity in the former, a
hypothesis that was not verified computationally. This issue forms the
focus of the present work, in which high-level calcns. and an explicit
detn. of the anharmonic force fields of both the anion and neutral are used
to predict the vibrational energy levels. Excellent agreement is achieved
for both 602 and 611, as both calcd. values fall within the exptl. error
bars.

 
 

 

 

 

 
 

Coupled-cluster studies of singlet propynylidene. Stanton, John F.;
Byun, K. Suzie. Institute for Theoretical Chemistry, The University of
Texas at Austin, Austin, TX, USA.

 

Mol. Phys. (1999), 96(4),
505-509. CODEN: MOPHAM ISSN: 0026-8976. Journal written in English.
CAN 130:281606 AN 1999:173572 CAPLUS (Copyright 2002 ACS)

Abstract

The equil. geometry of singlet propynylidene is studied with high level
coupled-cluster methods and moderate size to large basis sets. The calcns.
uniformly predict that the equil. structure is distorted from the C2v 1,3
biradical picture, with CC bond lengths that differ by approx. 0.1 .ANG..
Despite this, the barrier calcd. for automerization between the two equiv.
Cs min. on the potential surface is less than 1 kcal mol-1. Hence, it is
likely that exptl. studies of singlet propynylidene will be consistent with
the dynamically averaged C2v symmetry of the mol.

 
 

 

 

 

 
 

Structure and Energetics of Isomers of the Interstellar Molecule C5H.
Crawford, T. Daniel; Stanton, John F.; Saeh, Jamal C.; Schaefer, Henry F.,
III. Institute for Theoretical Chemistry Departments of Chemistry and
Biochemistry, The University of Texas, Austin, TX, USA.

 

J. Am. Chem.
Soc. (1999), 121(9), 1902-1911. CODEN: JACSAT ISSN: 0002-7863.
Journal written in English. CAN 130:267029 AN 1999:120249 CAPLUS
(Copyright 2002 ACS)

Abstract

High-level ab initio methods based on the coupled cluster approxn. have
been used to study properties of several isomers of the C5H radical, a mol.
of significant interest in radioastronomy. The three lowest-lying isomers
[the well-known linear form (1) as well as two ring-chain structures, HC2C3
(2) and C2C3H (3)] lie within 30 kcal/mol with isomer 2 approx. 5 kcal/mol
higher than the lowest-energy isomer 1. The computed rotational const. for
the linear isomer is within 0.7% of the value detd. in previous exptl.
analyses. Transition states formed via simple ring-opening mechanisms for
the interconversion of the linear and ring-chain isomers have also been
located; these lie ca. 27 and 31 kcal/mol above isomers 2 and 3, resp.,
indicating reasonable kinetic stability of these structures to
isomerization. The computed rotational consts. for these isomers should be
useful for lab. and astronomical observation of these species. In addn.,
four other min.-energy structures are found to lie somewhat higher in
energy. These isomers involve both three- and four-membered carbon rings,
as well as a bent-chain structure.

 
 

 

 

 

 
 

Anharmonic force fields from analytic CCSD(T) second derivatives: HOF and
F2O. Breidung, Jurgen; Thiel, Walter; Gauss, Jurgen; Stanton, John F..
Organisch-Chemisches Institut, Universitat Zurich, Zurich, Switz

 

.
J. Chem. Phys. (1999), 110(8), 3687-3696. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 130:257563 AN 1999:84156
CAPLUS (Copyright 2002 ACS)

Abstract

The recent implementation of analytic second derivs. for CCSD(T) (coupled
cluster theory with single and double excitations augmented by a
perturbational treatment of connected triple excitations) has been combined
with a numerical finite difference procedure to calc. cubic and
semi-diagonal quartic force fields. Computational details of this approach
are outlined. Applications are reported for HOF and F2O. The CCSD(T)
results are in excellent agreement with expt. and represent a substantial
improvement over the results obtained from MP2 (Moeller-Plesset
second-order perturbation theory).

 
 

 

 

 

 
 

Jahn-Teller Effect in VF3. Solomonik, Victor G.; Boggs, James E.;
Stanton, John F.. Institute for Theoretical Chemistry Departments of
Chemistry and Biochemistry, The University of Texas at Austin, Austin,
TX, USA.

 

J. Phys. Chem. A (1999), 103(7), 838-840. CODEN: JPCAFH
ISSN: 1089-5639. Journal written in English. CAN 130:201091 AN
1999:65888 CAPLUS (Copyright 2002 ACS)

Abstract

A computational study of the two lowest-lying electronic states, 3A'2 and
3E'', of the VF3 mol. is reported. Highly sophisticated methods were used,
including the coupled-cluster singles and doubles level augmented by a
perturbative correction for connected triple excitations and the
equation-of-motion coupled-cluster method in the singles and doubles
approxn. In contrast to previous theor. and exptl. data, the present
results predict the 3E'' state to be the ground electronic state of VF3 in
its trigonal planar D3h conformation. The A3A'2 state is 1300 cm-1 above
the X3E'' state. The Jahn-Teller (JT) effect in the X3E'' state was
studied. The three equiv. min. on the Jahn-Teller surface corresponding to
the 3B1 min. are 270 cm-1 below the D3h energy and sepd. by only 24 cm-1
barriers corresponding to 3A2 saddle points. The JT distortions of the D3h
structure are appreciable; at the 3B1 min. (C2v symmetry), the mol. has one
long V-F bond with R1e (V-F(1)) = 1.768 .ANG., two short bonds with R2e
(V-F(2)) = R3e (V-F(3)) = 1.748 .ANG., and the bond angle
.alpha.e(F(2)-V-F(3)) = 129.degree.. The available exptl. data on the VF3
mol. structure and spectra (gas-phase electron diffraction and matrix IR
and Raman spectroscopy) are discussed in light of the present results.

 
 

 

 

 

 
 

An estimation of the isomerization energy of acetylene. Stanton, John
F.; Gauss, Jurgen. Departments of Chemistry and Biochemistry, Institute
for Theoretical Chemistry, The University of Texas at Austin, Austin,
TX, USA.

 

J. Chem. Phys. (1999), 110(3), 1831-1832. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 130:167922 AN
1999:22379 CAPLUS (Copyright 2002 ACS)

Abstract

An accurate prediction of the adiabatic energy difference sepg. C2H2 and
CH2:C: is presented. The electronic-energy contribution is estd. on the
basis of CCSD(T) calcns. using .ltoreq.400 contracted Gaussian basis
functions together with extrapolation to the basis set limit; an addnl.
correction is applied to account for residual electron-correlation effects.
Zero-point vibrational corrections are based on quartic force fields of
both C2H2 and HC2:C: calcd. at the CCSD(T) level. Also included are
contributions for relativistic effects and the Born-Oppenheimer diagonal
correction. The isomerization energy is estd. as 15,200 .+-. 205 cm-1,
which is somewhat lower than the best previous theor. prediction.

 

 

 
1998 publications
 

 

 

 

Analytic UHF-CCSD(T) second derivatives. Implementation and application to
the calculation of the vibration-rotation interaction constants of NCO and
NCS. Szalay, Peter G.; Gauss, Juergen; Stanton, John F.. Institut
Physikalische Chemie, Universitaet Mainz, Mainz, Germany.


Theor. Chem. Acc. (1998), 100(1-4), 5-11. CODEN: TCACFW ISSN: 1432-881X.
Journal written in English. CAN 130:44691 AN 1998:776867 CAPLUS
(Copyright 2002 ACS)

Abstract

An implementation of analytic open-shell UHF-CCSD(T) 2nd derivs. is
presented. To demonstrate applicability and test the accuracy of the
UHF-CCSD(T) approach for the detn. of spectroscopical parameters,
vibration-rotation interaction consts. were calcd. for the ground (12.PI.)
and 1st electronically excited (12.SIGMA.) states of the NCO and NCS
radicals. In addn., harmonic vibrational frequencies for both states, the
Renner-Teller parameter for the ground state, as well as the 12.PI.
.fwdarw. 12.SIGMA. excitation energy are reported. While the computed
values are in good agreement with reliable exptl. data presented for NCO,
most of the data presented for NCS are predictions of quantities not well
known from expt.

 
 

 

 

 

 
 

Investigation of an asymmetric triple-excitation correction for
coupled-cluster energies. Crawford, T. Daniel; Stanton, John F..
Institute for Theoretical Chemistry, Departments of Chemistry and
Biochemistry, The University of Texas, Austin, TX, USA.

 

Int. J.
Quantum Chem. (1998), 70(4/5), 601-611. CODEN: IJQCB2 ISSN: 0020-7608.
Journal written in English. CAN 130:43508 AN 1998:749791 CAPLUS
(Copyright 2002 ACS)

Abstract

A correction for the effects of connected triple excitations to the
coupled-cluster singles and doubles energy is studied. The approach relies
on the fact that the ground-state coupled-cluster energy may be viewed as
an eigenvalue of an effective (similarity transformed) Hamiltonian with
assocd. left and right eigenvectors. Taking these as zeroth-order wave
functions and using a conventional partitioning of the bare electronic
Hamiltonian, the lowest order triple-excitation correction to the
correlation energy is found to have an asym. form that involves cluster
amplitudes as well as components of the left eigenvector. The popular (T)
correction may be viewed as an approxn. to the present approach, though the
latter is approx. a factor of 2 more expensive to compute. The method is
applied to a no. of difficult cases, including the harmonic vibrational
frequencies of ozone and the equil. bond length of N2. In addn., the
theory of analytic gradients for the method is outlined and some aspects
regarding its implementation are discussed.

 
 

 

 

 

 
 

Triple excitation effects in coupled-cluster calculations of
frequency-dependent hyperpolarizabilities. Gauss, Jurgen; Christiansen,
Ove; Stanton, John F.. Institut fur Physikalische Chemie, Universitat
Mainz, Mainz, Germany.

 

Chem. Phys. Lett. (1998), 296(1,2), 117-124.
CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
130:18634 AN 1998:670972 CAPLUS (Copyright 2002 ACS)

Abstract

The authors describe an implementation of an analytic scheme for the calcn.
of static and dynamical 1st hyperpolarizabilities at the CC3 level in the
framework of coupled-cluster response theory. Calcns. are reported for the
static, the 2nd harmonic generation, and the optical rectification
hyperpolarizabilities of FH. The results indicate the importance of triple
excitation effects for accurate theor. predictions of hyperpolarizabilities
and lend further support to previous contentions that an exptl. value for
the 2nd harmonic generation hyperpolarizabilities of FH should be
reconsidered.

 
 

 

 

 

 
 

Photochemical automerization of propadienylidene (H2C=C=C:). DePinto,
Jeffrey T.; Hodges, Jonathan A.; McMahon, Robert J.; Seburg, Randal A.;
Stanton, John F.. Department Chemistry, University Wisconsin, Madison,
WI, USA.

 

Book of Abstracts, 216th ACS National Meeting, Boston, August
23-27 (1998), PHYS-308. Publisher: American Chemical Society,
Washington, D. C CODEN: 66KYA2 Conference; Meeting Abstract written in
English. AN 1998:526747 CAPLUS (Copyright 2002 ACS)

Abstract

Singlet [13C]propadienylidene displays an intriguing photochem.
automerization reaction. At .lambda. > 444 nm, H2C=C=13C: (1a) and
H2C=13C=C: (1b) rapidly photoequilibrate. Computational results at the
CCSD(T)/ccpVTZ level of theory suggest that the equilibration occurs via a
conical intersection at the cyclopropyne geometry. Calcns. confirm that
the planar isomer of singlet cyclopropyne (2) represents the transition
state for the thermal interconversion of 1a and 1b. Unexpectedly, the
calcns. predict that the isomer of cyclopropyne contg. a tetrahedral carbon
atom lies ca. 7 kcal/mol higher in energy than the planar form. [Equation
Omitted].

 
 

 

 

 

 
 

The effect of triple excitations in coupled cluster calculations of
frequency-dependent polarizabilities. Christiansen, Ove; Gauss, Jurgen;
Stanton, John F.. Institut fur Physikalische Chemie, Universitat Mainz,
Mainz, Germany.

 

Chem. Phys. Lett. (1998), 292(4,5,6), 437-446.
CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
129:235756 AN 1998:504289 CAPLUS (Copyright 2002 ACS)

Abstract

Frequency-dependent polarizabilities have been implemented within the CC3
model. Comparison with full CI results shows that the triple excitation
effects included in CC3 lead to a significantly improved treatment for the
dispersion of the polarizability of CH+. For the refractivity and
polarizability anisotropy of N2, better agreement with expt. is found for
the CC3 results than for those obtained with the coupled cluster singles
and doubles model.

 
 

 

 

 

 
 

Polyclonal antibody catalytic variability. Stephens, David B.; Thomas,
Richard E.; Stanton, John F.; Iverson, Brent L. The Department of
Chemistry and Biochemistry, The University of Texas at Austin, Austin,
TX, USA.

 

Biochem. J. (1998), 332(1), 127-134. CODEN: BIJOAK ISSN:
0264-6021. Journal written in English. CAN 129:119470 AN
1998:370361 CAPLUS (Copyright 2002 ACS)

Abstract

We have performed a systematic variability study of polyclonal antibody
catalysis by using five rabbits immunized with the same hapten. Important
results from this work are the following. (1) Similarities were obsd. in
the catalytic polyclonal antibodies derived from all five rabbits. Four of
the five rabbits produced polyclonal samples that were nearly the same in
terms of catalytic activity, whereas the fifth rabbit, designated as rabbit
2, displayed a somewhat higher level of catalytic activity. The catalytic
activities (as kcat/kuncat) of these polyclonal samples were similar to
that from the best murine monoclonal antibody that had been previously
elicited by the same hapten. (2) Titer was not an accurate indicator of
polyclonal antibody catalytic activity. (3) A math. anal. to describe a
distribution of Michaelis-Menten catalysts was performed to help interpret
our results. (4) Kinetic anal. indicated that the binding parameters of
the different samples were remarkably homogeneous, because one or two
components were all that were required to fit the on-rate and off-rate data
satisfactorily. Interestingly, the most active catalytic polyclonal
sample, that from rabbit 2, displayed the slowest off-rate (so slow it
could not be measured) and thus the highest overall affinity. (5)
Catalytic anal. of eluted fractions of antibody from a substrate column
indicated that each polyclonal sample was also relatively homogeneous in
terms of catalytic parameters. The main conclusion of our study is that
for this hapten-animal system, the overall catalytic immune response is
relatively consistent at two levels. Consistent catalytic activity was
obsd. between the polyclonal samples elicited in the different animals, and
the elicited hapten-specific polyclonal antibodies were relatively
homogeneous in terms of binding and catalytic parameters within each
immunized animal. The obsd. similarities of the catalytic activity in the
different animals is surprising, because the immune response is based on
specific binding of antibodies to hapten. There is no known selective
pressure to maintain consistent levels of catalytic activity. Our results
can therefore be interpreted as providing evidence that for this hapten
there is a fixed relationship between hapten structure and catalytic
activity and/or consistent genetic factors that dominate the catalytic
immune response.

 
 

 

 

 

 
 

Anharmonicity in the ring stretching modes of diborane. Stanton, John
F.; Gauss, Jurgen. Departments of Chemistry , Institute for Theoretical
Chemistry, The University of Texas at Austin, Austin, TX, USA.

J.
Chem. Phys. (1998), 108(22), 9218-9220. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 129:59968 AN 1998:310165 CAPLUS
(Copyright 2002 ACS)

Abstract

Conspicuous differences between ab initio harmonic frequencies for the
.nu.6 and .nu.17 H bridge stretching modes of diborane and those calcd.
from an empirical harmonic force field have remained unexplained for nearly
20 yr. This work conclusively demonstrates that an unexpectedly large
anharmonicity effect is responsible for the discrepancy. Cubic and
semi-diagonal quartic force consts. obtained with coupled-cluster theory
and a reasonably large basis set are used to calc. fundamental frequencies.
For the bridge BH stretching (BHb) modes, all theor. values are within 15
cm-1 of the actual fundamental frequencies, while corresponding harmonic
frequencies are well above the empirical values. It thus appears that
BHb-stretching anharmonicity is considerably greater than previously
supposed, being roughly three times that (as measured by the magnitude of
1-.nu./.omega.) for terminal BH bonds. This probably is a general feature
of electron-deficient compds.

 
 

 

 

 

 
 

The equilibrium structure and fundamental vibrational frequencies of
dioxirane. Stanton, John F.; Lopreore, Courtney L.; Gauss, Jurgen.
Departments of Chemistry and Biochemistry, Institute for Theoretical
Chemistry, The University of Texas at Austin, Austin, TX, USA.

 

Journal of Chemical Physics (1998), 108(17), 7190-7196. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 129:15762 AN
1998:242916 CAPLUS (Copyright 2002 ACS)

Abstract

Complete sets of quadratic and cubic force consts. calcd. for 4 isotopomers
of dioxirane (I) are used to est. vibration-rotation interaction
contributions to obsd. values of rotational consts. (B''), thereby yielding
empirical ests. of the corresponding equil. values (Be). At the highest
levels of theory, least-squares refinements of at. coordinates to both the
empirical Be values and the assocd. isotope shifts yield consistent sets of
structural parameters. Recommended values are re(CO) = 1.3846 .+-. 0.0005
.ANG.; re(OO) = 1.5133 .+-. 0.0005 .ANG.; re(CH) = 1.0853 .+-. 0.0015 .ANG.
and .theta.e(HCH) = 117.03 .+-. 0.20.degree.. Semidiagonal quartic force
consts. (in the normal coordinate representation) also calcd. for I are
used to est. anharmonic contributions to the fundamental vibrational
frequencies. Arguments based on the latter set of results support those
made in a previous theor. study and clearly show that 2 IR features
assigned to dioxirane in a matrix-isolation expt. must be due to a
different carrier.

 
 

 

 

 

 
 

A coupled cluster study of the 11A1g and 11B2u states of benzene.
Christiansen, Ove; Stanton, John F.; Gauss, Jurgen. Department of
Chemistry, Arhus University, Aarhus, Den.

 

J. Chem. Phys. (1998),
108(10), 3987-4001. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 128:275336 AN 1998:148957 CAPLUS (Copyright 2002
ACS)

Abstract

A theor. investigation of the equil. structures and harmonic frequencies of
the 11A1g and 11B2u states of benzene is presented. The performance of
coupled cluster singles (CCS), the recently proposed CC2 model, and coupled
cluster singles and doubles (CCSD) is compared. The CC2 ground and excited
states frequencies are a significant improvement of the CCS results and are
relatively close to the CCSD results. A comparative anal. of the
vibrations in the two electronic states of both C6H6 and C6D6 is presented.
The reliability of predicted shifts in harmonic frequencies between the
two states and isotopic shifts is estd. on the basis of the convergence in
the CCS, CC2, and CCSD hierarchy of models, and through comparison with
related theor. work. The shifts are used in a crit. comparison with expt.
Inconsistent exptl. assignments have been scrutinized on the basis of the
predicted shifts. The complete harmonic force fields are given for both
states.

 

 

 

 
1997 publications
 

 

 

 

A coupled cluster study of the 11A1g and 11B2u states of benzene.
Christiansen, Ove; Stanton, John F.; Gauss, Jurgen. Department of
Chemistry, Arhus University, Aarhus, Den.

 

J. Chem. Phys. (1998),
108(10), 3987-4001. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 128:275336 AN 1998:148957 CAPLUS (Copyright 2002
ACS)

Abstract

A theor. investigation of the equil. structures and harmonic frequencies of
the 11A1g and 11B2u states of benzene is presented. The performance of
coupled cluster singles (CCS), the recently proposed CC2 model, and coupled
cluster singles and doubles (CCSD) is compared. The CC2 ground and excited
states frequencies are a significant improvement of the CCS results and are
relatively close to the CCSD results. A comparative anal. of the
vibrations in the two electronic states of both C6H6 and C6D6 is presented.
The reliability of predicted shifts in harmonic frequencies between the
two states and isotopic shifts is estd. on the basis of the convergence in
the CCS, CC2, and CCSD hierarchy of models, and through comparison with
related theor. work. The shifts are used in a crit. comparison with expt.
Inconsistent exptl. assignments have been scrutinized on the basis of the
predicted shifts. The complete harmonic force fields are given for both
states.

 
 

 

 

 

 
 

Why CCSD(T) works: a different perspective. Stanton, John F..
Departments of Chemistry and Biochemistry, Institute for Theoretical
Chemistry, The University of Texas at Austin, Austin, TX 78712, USA

.
Chem. Phys. Lett. (1997), 281(1,2,3), 130-134. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 128:145516 AN
1997:811738 CAPLUS (Copyright 2002 ACS)

Abstract

The original motivation for the CCSD(T) method was an attempt to treat the
effects of triply-excited determinants on both single and double excitation
operators on an equal footing. Hence, conventional analyses based on
perturbation theory cannot satisfactorily explain why the particular
fifth-order term included in CCSD(T) should be chosen over a no. of other
possibilities. This work demonstrates that the terms appearing in CCSD(T)
can be justified if one takes the biorthogonal representation of the CCSD
state as the zeroth-order wavefunction. This perspective provides some
addnl. insight as to why the method works so well in practice.

 
 

 

 

 

 
 

Hartree-Fock orbital instability envelopes in highly correlated
single-reference wave functions. Crawford, T. Daniel; Stanton, John F.;
Allen, Wesley D.; Schaefer, Henry F., III. Department of Chemistry,
Center for Computational Quantum Chemistry, The University of Georgia,
Athens, GA, USA.

 

J. Chem. Phys. (1997), 107(24), 10626-10632.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
128:145507 AN 1997:807605 CAPLUS (Copyright 2002 ACS)

Abstract

The effects of Hartree-Fock orbital instabilities on force const.
predictions at both Hartree-Fock and correlated levels of theory are
investigated. Due to the quadratic dependence of the second deriv. of
correlated energies on the orbital rotation parameters, anomalous force
const. singularities enveloped by "instability volcanoes" are given by the
single-ref. correlation methods examd. here. Infinite-order
coupled-cluster methods are indeed affected by the ref. instability, but
over a rather small region of the potential surface, whereas perturbative
triples corrections tend to widen the coupled-cluster volcano.
Finite-order many-body perturbation theory yields very wide volcanoes, and
corresponding predictions of vibrational spectra may be seriously
compromised if the geometry of interest lies at all in the vicinity of an
instability in the ref. determinant.

 
 

 

 

 

 
 

Analytic evaluation of second derivatives of the energy: computational
strategies for the CCSD and CCSD(T) approximations. Stanton, John F.;
Gauss, Jurgen. Departments of Chemistry and Biochemistry, Institute for
Theoretical Chemistry, University of Texas at Austin, Austin, TX, USA

 

.
Recent Adv. Comput. Chem. (1997), 3(Recent Advances in Coupled-Cluster
Methods), 49-79. CODEN: RACCF6 Journal written in English. CAN
128:39731 AN 1997:747515 CAPLUS (Copyright 2002 ACS)

Abstract

Various strategies were analyzed for calcg. CCSD and CCSD(T) second
derivs., and some potential benefits were found regarding sym. and asym.
approaches. For CCSD 2 distinct strategies can be followed and each of
them is advantageous in certain situations (on the basis of CPU
considerations only, the sym. approach is preferred; the asym. strategy
might be advantageous because of its less drastic disk space requirements).
The asym. formulation provides the most sensible and computationally
efficient strategy for CCSD(T) second deriv. calcns. regardless of the mol.
or the second-order property in question.

 
 

 

 

 

 
 

Structures and Stabilities of C5H2 Isomers: Quantum Chemical Studies.
Seburg, Randal A.; McMahon, Robert J.; Stanton, John F.; Gauss, Juergen.
Department of Chemistry, University of Wisconsin, Madison, WI, USA

 

.
J. Am. Chem. Soc. (1997), 119(44), 10838-10845. CODEN: JACSAT ISSN:
0002-7863. Journal written in English. CAN 127:247738 AN
1997:667222 CAPLUS (Copyright 2002 ACS)

Abstract

Five isomers of the carbon-rich mol. C5H2 are investigated computationally,
using methods based on the coupled-cluster approxn. All of these
structures are related to isomers of C3H2 via substitution of hydrogen by
ethynyl or attachment of a C2 fragment to a carbene center. The two most
stable forms of C5H2 are linear triplet pentadiynylidene and singlet
ethynylcyclopropenylidene . Both of these isomers have been obsd. in the
lab., as has a third-the cumulene carbene pentatetraenylidene -which is
predicted to lie about 15 kcal/mol above the linear triplet. Two other
isomers are also studied: ethynylpropadienylidene and
3-(didehydrovinylidene)cyclopropene . Both are found to lie less than 25
kcal/mol above the most stable form of C5H2 and to possess rather large
dipole moments. Predictions for the harmonic vibrational frequencies of
12C and mono-13C isotopomers, IR intensities, and rotational consts. are
also presented. These should assist efforts to identify these mols. in the
lab. and in the interstellar medium.

 
 

 

 

 

 
 

Analytic CCSD(T) second derivatives. Gauss, Juergen; Stanton, John F..
Institut fuer Physikalische Chemie, Universitaet Mainz, Mainz, Germany

 

.
Chem. Phys. Lett. (1997), 276(1,2), 70-77. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 127:306999 AN
1997:616365 CAPLUS (Copyright 2002 ACS)

Abstract

A general-purpose implementation of analytic CCSD(T) 2nd derivs. is
presented. Its applicability is demonstrated by calcns. of
vibration-rotation interaction consts. for the astrophys. important mol.
cyclopropenylidene (C3H2), in which the required cubic force consts. were
detd. by numerical differentiation of anal. evaluated 2nd derivs. of the
energy.

 
 

 

 

 

 
 

Electron correlation effects on the calculated 13C NMR spectra of vinyl
cations. Gauss, Jurgen; Stanton, John F.. Lehrstuhl Theoretische
Chem., Inst. Physikalische Chem., Univ. Karlsruhe, Karlsruhe, Germany.

 


THEOCHEM (1997), 398-399 73-80. CODEN: THEODJ ISSN: 0166-1280.
Journal written in English. CAN 127:292710 AN 1997:611761 CAPLUS
(Copyright 2002 ACS)

Abstract

A study of calcd. 13C NMR chem. shifts in vinyl cations is presented. The
sensitivity of predicted isotropic shifts to correlation, basis set and
geometry effects is explored. In order to obtain accurate ests. that are
reasonably well converged with respect to further improvements in theory,
it appears that the CCSDT method must be used with a basis of triple-zeta
plus polarization quality on the carbon atoms. Second-order many-body
perturbation theory performs adequately for all carbons except for that
bearing the formal pos. charge, while the SCF approxn. cannot be relied
upon to predict even the correct qual. ordering in the spectrum. Various
extrapolation schemes that assume an approx. additivity of basis set and
correlation effects are explored.

 
 

 

 

 

 
 

The ~C 2A2 excited state of NO2: evidence for a Cs equilibrium structure
and a failure of some spin-restricted reference wavefunctions.
Crawford, T. Daniel; Stanton, John F.; Szalay, Peter G.; Schaefer, Henry
F., III. Center for Computational Quantum Chemistry, Dep. Chem., Univ.
Georgia, Athens, GA, USA.

 

J. Chem. Phys. (1997), 107(7),
2525-2528. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 127:269642 AN 1997:547630 CAPLUS (Copyright 2002 ACS)

Abstract

The geometry and harmonic vibrational frequencies of the ~C 2A2 electronic
state of nitrogen dioxide were detd. using coupled cluster (CC) theory in
conjunction with large basis sets. The min. energy C2v-constrained N-O
bond distance predicted by the highest level of theory is significantly
shorter than the exptl. inferred value, and it is unlikely that further
refinement in the theor. treatment would reduce the discrepancy. However,
this work suggests that a prevailing assumption of C2v symmetry may be
incorrect; the equil. structure of the ~C state may instead have Cs
symmetry. And contrary to expectation, CC calcns. based on variationally
optimal spin-unrestricted or spin-restricted ref. determinants give qual.
different results for this system, unless very high levels of electron
correlation are included in the calcn.

 
 

 

 

 

 
 

Computational Evidence for a Metastable Polar Isomer of Beryllium
Borohydride. Saeh, Jamal C.; Stanton, John F.. Institute for
Theoretical Chemistry Departments of Chemistry and Biochemistry,
University of Texas, Austin, TX, USA.

 

J. Am. Chem. Soc. (1997),
119(31), 7390-7391. CODEN: JACSAT ISSN: 0002-7863. Journal written in
English. CAN 127:153179 AN 1997:510210 CAPLUS (Copyright 2002
ACS)

Abstract

Intuitive arguments suggesting that a stable polar isomer of BeB2H8 might
be formed by interaction of cationic BeH and the anion of diborane(7) are
tested computationally. A species contg. three Be-H-B bridge bonds with Cs
symmetry represents a true local min. on the potential energy surface at
the electron-correlated level of theory. Other stationary points are found
that represent landmarks on a rather complicated pathway for intramol.
hydrogen exchange via rotation of BH3 units. Although the Cs isomer of
BeB2H8 lies roughly 20 kcal mole-1 above other local min. with linear
B-Be-B arrangements, no low-energy pathway for conversion to the more
stable isomers is apparent. Given that the cryst. structure of BeB2H8 is
polymeric with decidedly nonlinear B-Be-B contacts, it is plausible that
the triangular species studied here might exist as a metastable
intermediate. Hence, this isomer provides a basis for rationalizing the
dipole moment expts. of a quarter-century ago that helped to make the mol.
structure of this mol. such an enigmatic and frustrating problem.

 
 

 

 

 

 
 

Theoretical study of electronically excited cis- and trans-glyoxal.
Stanton, John F.; Gauss, Jurgen. Institute for Theoretical Chemistry,
Departments of Chemistry and Biochemistry, The University of Texas at
Austin, Austin, TX, USA.

 

Spectrochim. Acta, Part A (1997), 53A(8),
1153-1162. CODEN: SAMCAS ISSN: 0584-8539. Journal written in English.
CAN 127:161353 AN 1997:458860 CAPLUS (Copyright 2002 ACS)

Abstract

The equation-of-motion coupled cluster method for excitation energies in
the singles and doubles approxn. (EOMEE-CCSD) is applied to an
investigation of the structure and harmonic frequencies of planar
conformers of glyoxal in their first excited singlet states. For the
trans-isomer, agreement between calcd. harmonic frequencies and obsd.
fundamentals is generally satisfactory, although the theor. values are
slightly more than 10% too high for the carbonyl stretching modes.
Parallel calcns. of the corresponding ground state properties allow for an
empirical prediction of the excited state frequencies in which calcd.
differences in normal-mode frequencies are simply added to the ground state
fundamentals. Ests. made by this procedure are within 20 cm-1 of the
actual positions for all modes except the carbonyl stretches and the
in-phase CH stretch, for which the exptl. assignment is uncertain. For the
cis isomer, the results presented here should be useful in anal. of future
expts. Significantly, the exptl. inferred dipole moment of 4.8 Debye for
this isomer appears to be in error.

 
 

 

 

 

 
 

Structures, Automerizations, and Isomerizations of C3H2 Isomers.
Seburg, Randal A.; Patterson, Eric V.; Stanton, John F.; McMahon, Robert J.
Department of Chemistry, University of Wisconsin, Madison, WI, USA.

 


J. Am. Chem. Soc. (1997), 119(25), 5847-5856. CODEN: JACSAT ISSN:
0002-7863. Journal written in English. CAN 127:33758 AN 1997:403021
CAPLUS (Copyright 2002 ACS)

Abstract

A combination of expt. and theory provides insight into the structure and
rearrangements of various C3H2 isomers. Photolysis of [13C]diazopropynes
HC.tplbond.CC*(:N2)H, HC*.tplbond.CC(:N2)H, HC.tplbond.C*C(:N2)H (* = 13C)
under matrix isolation conditions affords C3H2 isomers contg. a single
13C-label. With the aid of computed vibrational frequencies and
intensities (CCSD(T)/cc-pVTZ), the seven 13C-isotopomers of triplet
propynylidene HC.bul.:C:C*.bul.H/HC.bul.:C*:C.bul.H, singlet
propadienylidene H2C*:C:C:/H2C:C:C*:/H2C:C*:C:, and singlet
cyclopropenylidene I/II are readily distinguished by IR spectroscopy.
Monitoring the distribution of the 13C-label during photolysis at either
.lambda. = 313 .+-. 10 nm or .lambda. > 444 nm reveals the involvement of
two photochem. automerization processes. At .lambda. = 313 .+-. 10 nm,
triplet propynylidene and singlet cyclopropenylidene photoequilibrate. The
interconversion does not occur by a simple ring closure/ring opening
mechanism, as hydrogen migration accompanies the interconversion. At
.lambda. > 444 nm, H2C:C:C*: and H2C:C*:C: (2b and 2c, resp.) rapidly
equilibrate. Various lines of evidence suggest that the equilibration
occurs through a cyclopropyne intermediate or transition state.
Computational results confirm that the planar isomer of singlet
cyclopropyne (III, C2v) is the transition state for the interconversion of
2b and 2c. Unexpectedly, the calcns. predict that the isomer of this
compd. contg. a tetrahedral carbon atom (IV, C2v) lies ca. 7 kcal/mol
higher in energy than the planar form.

 
 

 

 

 

 
 

A simple correction to final state energies of doublet radicals described
by equation-of-motion coupled cluster theory in the singles and doubles
approximation. [Erratum to document cited in CA125:68133]. Stanton,
John F.; Gauss, Juergen. Institue Theoretical Chemistry, University
Texas Austin, Austin, TX, USA.

 

Theor. Chim. Acta (1997), 95(3-4),
97-98. CODEN: TCHAAM ISSN: 0040-5744. Journal written in English.
CAN 127:9279 AN 1997:344421 CAPLUS (Copyright 2002 ACS)

Abstract

Equations (18), (19), (20), (21), (27), (28), (29) and Table 1 are cor.
EOMIP-CCSD* final state energies are more uniformly pos. than the erroneous
values documented in the original, and are in excellent agreement with the
UHF-CCSD energies. Therefore, it is not true (as stated in the original
paper) that EOMIP-CCSD* energies tend to lie closer to UHF-CCSD(T) than
UHF-CCSD. The principal conclusions of the paper are still valid.

 
 

 

 

 

 
 

Electronic Spectrum of Propadienylidene (H2C:C:C:). Stanton, John F.;
DePinto, Jeffrey T.; Seburg, Randal A.; Hodges, Jonathan A.; McMahon,
Robert J. Institute for Theoretical Chemistry, University of Texas,
Austin, TX, USA.

 

J. Am. Chem. Soc. (1997), 119(2), 429-430. CODEN:
JACSAT ISSN: 0002-7863. Journal written in English. CAN 126:103800
AN 1997:112967 CAPLUS (Copyright 2002 ACS)

Abstract

The electronic absorption spectrum of propadienylidene (H2C:C:C:) (1) has
been obtained under conditions of argon matrix isolation at 10 K. Calcns.
of the ground state and the three lowest singlet excited states of 1 using
the equation-of-motion coupled-cluster method have been used to interpret
the spectrum. The .hivin.A1A2 .rarw. .hivin.X1A1 transition is only
vibronically allowed, and gives rise to a series of weak absorptions
beginning near 717 nm (1.73 eV). The .hivin.B1B1 .rarw. .hivin.X1A1
transition (535 nm, 2.32 eV) and the .apprx.C1A1 .rarw. .hivin.X1A1
transition (259 nm, 4.79 eV) display vibrational progressions assocd. with
C-C stretching vibrations of ca. 1000 cm-1 and 900 cm-1, resp. The
presence of high-structured visible absorptions in the spectrum and the
confirmed presence of propadienylidene and its homologs in the interstellar
medium suggest that this family of compds. might be responsible for some of
the mysterious diffuse interstellar bands.

 
 

 

 

 

 
 

Nuclear magnetic shielding constants in the CC2 model. Christiansen,
Ove; Gauss, Juergen; Stanton, John F.. Department of Chemistry, Arhus
University, Aarhus, Den.

 

Chem. Phys. Lett. (1997), 266(1,2), 53-60.
CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
126:271237 AN 1997:109833 CAPLUS (Copyright 2002 ACS)

Abstract

Test calcns. of nuclear magnetic shielding consts. in the CC2 model were
performed using the gauge-including AO approach. Abs. shielding consts.
are reported for representative 1st-row hydrides, a few multiply bonded
mols. and some challenging cases. The performance of CC2 is analyzed by
comparison with exptl. data and results from calcns. employing more
sophisticated treatments of electron correlation. In most cases, CC2
shieldings and chem. shifts are close to those obtained at 2nd-order
perturbation theory, despite the fact that the CC2 model includes an
approx. treatment of orbital relaxation effects at the correlated level.

 
 

 

 

 

 
 

Vibrational assignment of the S1 fluorescence excitation spectrum of formyl
fluoride. Crane, Jason C.; Nam, Hakhyun; Beal, Harry P.; Clauberg,
Horst; Choi, Young S.; Moore, C. Bradley; Stanton, John F.. Dep. Chem.,
Univ. California, Berkeley, CA, USA.

 

J. Mol. Spectrosc. (1997),
181(1), 56-66. CODEN: JMOSA3 ISSN: 0022-2852. Journal written in
English. CAN 126:178371 AN 1997:34528 CAPLUS (Copyright 2002
ACS)

Abstract

The S1 fluorescence excitation spectrum of jet-cooled formyl fluoride, HFCO
and DFCO, was recorded from 37,500 to 40,250 cm-1 at 0.20 cm-1 resoln.
Vibrational bands were assigned primarily to progressions in the
Franck-Condon active modes .nu.2, .nu.5, and .nu.6. Tunneling splittings
were obsd. in the .nu.6 progression and fit to a model double-well
potential Hamiltonian. Vibrational energies were fit to a Dunham expansion
modified to account for the inversion doubling in the .nu.6 progression.
Best fit spectroscopic consts. are reported including harmonic frequencies
and anharmonic consts. S0 .fwdarw. S1 Franck-Condon factors were calcd.
with the Duschinsky rotation included and support the vibrational
assignment. The S1 geometry and vibrational potential inferred from the
present anal. are compared to ab initio results.

 

 

 
1996 publications
 

 

 

 

Ab initio MO studies on the photodissociation of C2H2 from the S1 (1Au)
state. Non-adiabatic effects and S-T interaction. Cui, Qiang; Morokuma,
Keiji; Stanton, John F.. Cherry L. Emerson Center for Scientific
Computation and Department of Chemistry, Emory University, Atlanta, USA.

 


Chem. Phys. Lett. (1996), 263(1,2), 46-53. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 126:96741 AN 1996:726772
CAPLUS (Copyright 2002 ACS)

Abstract

Ab initio MO calcns. at the EOM-CCSD and CASSCF level have been carried out
for several low lying electronic states of C2H2. Many min. on the seam of
crossing and transition states are located. For photodissocn. from the S1
state, a non-adiabatic path involving a S1.fwdarw.S2 transition followed by
a S2.fwdarw.S0 crossing which produces mainly C2H(X 2.SIGMA.+) is preferred
at low energy to the S1 adiabatic path producing mainly C2H(A2.PI.), in
agreement with exptl. propensity. A min. on the seam of crossing between
S1 and T3 is found. T3 may be a strong perturbing state in the ZAC
spectrum of S1.

 
 

 

 

 

 
 

CCSD(T) calculation of NMR chemical shifts: consistency of calculated and
measured 13C chemical shifts in the 1-cyclopropylcyclopropylidenemethyl
cation. Stanton, John F.; Gauss, Juergen; Siehl, Hans-Ullrich.
Institute for Theoretical Chemistry, Departments of Chemistry and
Biochemistry, The University of Texas at Austin, Austin, TX, USA.

 


Chem. Phys. Lett. (1996), 262(3,4), 183-186. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 126:89021 AN 1996:663083
CAPLUS (Copyright 2002 ACS)

Abstract

The source of conspicuous disagreement between theory and expt. for the
chem. shift of C.alpha. in the 1-cyclopropylcyclopropylidenemethyl cation
is identified as an inadequate treatment of electron correlation effects in
a previous theor. study. When the sophisticated CCSD(T) method is used,
all calcd. 13C chem. shifts agree with the exptl. values to within 2.2 ppm.
These findings undermine the basis of an earlier suggestion that the
selectively poor performance of theory for the C.alpha. nucleus can be
attributed to environmental effects in which solvent interactions
significantly perturb the geometrical structure of the cation.

 
 

 

 

 

 
 

Application of the equation-of-motion coupled cluster method to excited
state potential surfaces. Stanton, John F.. Institute Theoretical
Chemistry, University Texas, Austin, TX, USA.

 

Book of Abstracts,
212th ACS National Meeting, Orlando, FL, August 25-29 (1996),
PHYS-348. Publisher: American Chemical Society, Washington, D. C CODEN:
63BFAF Conference; Meeting Abstract written in English. AN 1996:415760
CAPLUS (Copyright 2002 ACS)

Abstract

The equation-of-motion (or linear response) coupled cluster (EOMCC) method
originally formulated by Monkhorst has been used in recent years to study
excited states of several closed shell mols. Analytic gradient strategies
developed and implemented for this method have facilitated its application
to the study of excited state potential surtaces, particularly in the
Franck-Condon region (where the method works particularly well). This talk
will include an overview of the basic theory and its relationship to the
more familiar method of CI, a discussion of its computational scaling
properties, and selected chem. applications.

 
 

 

 

 

 
 

The first excited singlet state of s-tetrazine: A theoretical analysis of
some outstanding questions. Stanton, John F.; Gauss, Juergen. Dep.
Chemistry, Univ. Texas Austin, Austin, TX, USA.

 

J. Chem. Phys.
(1996), 104(24), 9859-9869. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 125:113954 AN 1996:375857 CAPLUS
(Copyright 2002 ACS)

Abstract

The equation-of-motion coupled cluster method for excited electronic states
(EOMEE-CC) is applied to study the structure and selected properties of the
first excited singlet state of s-tetrazine. Adiabatic S1.rarw.S0
excitation energies obtained with large basis sets contg. up to 270
functions are uniformly somewhat above the exptl. 0-0 value of 2.238 eV,
but nevertheless are the most accurate calcns. reported to date for this
quantity. The equil. geometry of S1 predicted in this study is in
excellent agreement with another high-level calcn., and moreover is quant.
consistent with both the intensity of vibrational progressions obsd. in
absorption and measured rotational consts. for S1. The EOMEE-CC harmonic
force field of S1 is the first to satisfactorily describe the low frequency
in-plane b3g bending mode, notably its marked redn. in frequency upon
excitation and characteristic pos. anharmonicity. Both of these effects
are due to a strong second-order Jahn-Teller interaction between S1 and the
nominal S2(1Au) state, which is also investigated superficially in this
work. Finally, results presented for the static dipole polarizabilities of
the S1 state join others in calling for a reinvestigation of the exptl.
detd. parameters.

 
 

 

 

 

 
 

A simple correction to final state energies of doublet radicals described
by equation-of-motion coupled cluster theory in the singles and doubles
approximation. Stanton, John F.; Gauss, Juergen. Institute for
Theoretical Chemistry, The University of Texas at Austin, Austin, TX,
USA.

 

Theor. Chim. Acta (1996), 93(5), 303-313. CODEN: TCHAAM ISSN:
0040-5744. Journal written in English. CAN 125:68133 AN 1996:327495
CAPLUS (Copyright 2002 ACS)

Abstract

A computationally inexpensive energy correction is suggested for radicals
described by the equation-of-motion coupled cluster method for ionized
states in the singles and doubles approxn. (EOMIP-CCSD). The approach is
primary intended for doublet states that are qual. described by Koopman's
approxn. Following a strategy similar to those used in multireference
coupled cluster theory, the proposed correction accounts for all
correlation effects through 3rd order in perturbation theory and also
includes selected contributions to higher-order energies. As an initial
test of the numerical performance of the method, total energies and energy
splittings are calcd. for some small prototype radicals.

 
 

 

 

 

 
 

The ~A 1Au state and the T2 potential surface of acetylene: Implications
for triplet perturbations in the fluorescence spectra of the A state.
Sherrill, C. David; Vacek, George; Yamaguchi, Yukio; Schaefer, Henry F.,
III; Stanton, John F.; Gauss, Jurgen. Center Computational Quantum
Chemistry, Univ. Georgia, Athens, GA, USA.

 

J. Chem. Phys. (1996),
104(21), 8507-8515. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 125:85875 AN 1996:327205 CAPLUS (Copyright 2002
ACS)

Abstract

The cis-trans isomerization reaction on the T2 surface of acetylene and the
lowest excited singlet state of acetylene, ~A1Au, are investigated by ab
initio electronic structure theory. We report optimized geometries, dipole
moments, and harmonic vibrational frequencies of stationary points and
adiabatic energy differences between them using basis sets as large as
triple-.zeta. plus double polarization with higher angular momentum
functions, TZ(2d,2pd), and theor. methods up to coupled-cluster singles and
doubles with a perturbative triples correction [CCSD(T)] and the
equation-of-motion coupled-cluster method (EOM-CCSD). Our theor.
predictions should aid the interpretation of observations from a series of
recent spectroscopic studied involving excited electronic states of
acetylene. In particular, the present theor. results rule out several
possible explanations for the anomalous sudden increase in detectable
Zeeman anticrossings reported by Dupre et al. [Chem. Phys. 152, 293
(1991)].

 
 

 

 

 

 
 

Pseudorotational interconversion of the 2A1 and 2B2 states of HCOO.
Stanton, John F.; Kadagathur, Narayani S. Institute for Theoretical
Chemistry, Departments of Chemistry and Biochemistry, The University of
Texas at Austin, Austin, TX, USA.

 

J. Mol. Struct. (1996), 376
469-74. CODEN: JMOSB4 ISSN: 0022-2860. Journal written in English.
CAN 124:270977 AN 1996:142956 CAPLUS (Copyright 2002 ACS)

Abstract

The lowest potential energy surface for the .sigma. formyloxyl radical
(.sigma.-HCOO) is investigated with an equation-of-motion coupled cluster
technique and basis sets that contain up to 195 contracted Gaussian
functions. The 2B2 and 2A1 states of sym. (C2v) .sigma.-HCOO both lie on
this surface and can be interconverted adiabatically by transporting the
nuclei through nonsym., but planar, intermediate geometries. In accord
with previous high-level calcns., the potential energy surface that governs
this pseudorotation process is predicted to be extremely flat. While the
2A1 state is found to be the highest point on the interconversion pathway
when basis sets of modest size are used, this sym. form is preferentially
stabilized by basis set augmentation. Calcns. with very large basis sets
suggest that both the 2A1 and 2B2 C2v structures are local min. on the
surface, sepd. by a barrier of about 500 cm-1. These results are in qual.
agreement with conclusions reached in a recent electron photodetachment
expt.

 
 

 

 

 

 
 

Perturbative treatment of triple excitations in coupled-cluster
calculations of nuclear magnetic shielding constants. Gauss, Juergen;
Stanton, John F.. Lehrstuhl fuer Theoretische Chemie, Universitaet
Karlsruhe, Karlsruhe, Germany.

 

J. Chem. Phys. (1996), 104(7),
2574-83. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 124:192089 AN 1996:98538 CAPLUS (Copyright 2002 ACS)

Abstract

A theory for the calcn. of nuclear magnetic shielding consts. at the
coupled-cluster singles and doubles level augmented by a perturbative
correction for connected triple excitations (CCSD(T)) was developed and
implemented. The approach, which is based on the gauge-including AO (GIAO)
ansatz, is illustrated by several numerical examples. These include a
comparison of CCSD(T) and other highly correlated methods with full CI for
the BH mol., and a systematic comparison with expt. for HF, H2O, NH3, CH4,
N2, CO, HCN, and F2. The results demonstrate the importance of triple
excitations in establishing quant. accuracy. Finally, the ability of
GIAO-CCSD(T) to make accurate predictions for difficult cases is explored
in calcns. for formaldehyde (CH2O), diazomethane (CH2NN), and ozone (O3).

 

 

1995 publications

 

 

 

Many-body methods for excited state potential energy surfaces. II. Analytic
second derivatives for excited state energies in the equation-of-motion
coupled cluster method. Stanton, John F.; Gauss, Jurgen. Inst.
Theor. Chem., Univ. Texas at Austin, Austin, TX, USA.

 

J. Chem. Phys.
(1995), 103(20), 8931-43. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 124:15866 AN 1995:954255 CAPLUS
(Copyright 2002 ACS)

Abstract

Two distinct theor. formalisms are developed for evaluating second derivs.
of the energy anal. within the equation-of-motion coupled cluster approxn.
for excited electronic states (EOMEE-CC). In the first approach, both
perturbations are treated equivalently. In the alternative formulation,
the final operator expression is not sym. with respect to interchange of
the perturbations, and calcn. of the second deriv. requires that four
systems of linear equations be solved for the first-order response of wave
function parameters. However, only two systems need to be solved when the
sym. strategy is followed. While the sym. approach superficially appears
to be both more elegant and better suited for practical calcns., anal.
shows that the former assertion is open to question and the latter only
conditionally true. In particular, the asym. formulation is shown to be
the preferred choice for all cases in which a large no. of perturbations is
involved. This is a rather important conclusion that holds not only for
the EOMEE-CC method, but also for CC treatments of the electronic ground
state and their finite-order many-body perturbation theory approxns.

 
 

 

 

 

 
 

A comparison of single reference methods for characterizing stationary
points of excited state potential energy surfaces. Stanton, John F.;
Gauss, Juergen; Ishikawa, Naoto; Head-Gorgon, Martin. Inst. Theor.
Chem., Univ. Texas, Austin, TX, USA.

 

J. Chem. Phys. (1995),
103(10), 4160-74. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 123:209366 AN 1995:800911 CAPLUS (Copyright 2002
ACS)

Abstract

The accuracy of geometries, vibrational frequencies and dipole moments of
stationary points on excited state potential energy surfaces is assessed
for three single ref. excited state theories - CI (CIS), a perturbative
doubles correlation correction to CIS, termed CIS(D), and
equation-of-motion coupled cluster theory with single and double
substitutions (EOM-CCSD). Two groups of systems are studied: the diat.
mols. H2, BH, BF, C2, CO, and N2; and the lowest singlet excited states of
ammonia, formaldehyde and acetylene. The calcns. demonstrate that CIS
systematically underestimates bond lengths and overestimates frequencies
and dipole moments, a pattern often assocd. with the Hartree-Fock method
for ground states. CIS(D) fails to provide a systematic improvement to CIS
for all geometries and frequencies, often overestimating correlation
corrections. EOM-CCSD, by contrast, performs significantly better than CIS
for all properties considered.

 
 

 

 

 

 
 

Coupled-cluster calculations of nuclear magnetic resonance chemical shifts.
Gauss, Juergen; Stanton, John F.. Lehrstuhl Theoretische Chemie,
Inst. Physikalische Chemie, Univ. Karlsruhe, Karlsruhe, Germany.

 


J.Chem. Phys. (1995), 103(9), 3561-77. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 123:304926 AN 1995:796036 CAPLUS
(Copyright 2002 ACS)

Abstract

Theory and implementation of the gauge-including AO (GIAO) ansatz for the
gauge-invariant calcn. of NMR chem. shifts are described for the
coupled-cluster singles and doubles (CCSD) approach. Results for the
shielding consts. of the hydrides HF, H2O, NH3, and CH4 as well as for a
few multiply bonded systems such as CO, N2, and HCN demonstrate the
importance of higher-order correlation corrections, as good agreement with
expt. is only obtained at the CCSD level and to some extent at partial
4th-order many-body perturbation theory [SDQ-MBPT(4)] with the latter
slightly overestimating correlation effects due to single and double
excitations. For relative chem. shifts, GIAO-CCSD calcns. provide in
difficult cases (e.g., CO and CF4) more accurate results than previous
GIAO-MBPT(2) calcns. But, it seems that it is often more important to
include rovibrational effects (as well as possible mol.-solvent
interactions) than higher-order correlation corrections. Despite that,
GIAO-CCSD proves to be a powerful tool for the accurate calcn. of NMR chem.
shifts. Its capabilities as well as its limitations are demonstrated in
shielding calcns. for formaldehyde, diazomethane, and ozone. At least for
the latter, the description provided by the CCSD ansatz is not sufficient
and even higher excitations need to be considered.

 
 

 

 

 

 
 

Analytic energy derivatives for the equation-of-motion coupled-cluster
method: algebraic expressions, implementation and application to the S1
state of HFCO. Stanton, John F.; Gauss, Juergen. Dep. Chemsitry,
Univ. Texas, Austin, TX, USA.

 

Theor. Chim. Acta (1995), 91(5/6),
267-89. CODEN: TCHAAM ISSN: 0040-5744. Journal written in English.
CAN 123:179696 AN 1995:722456 CAPLUS (Copyright 2002 ACS)

Abstract

The general theory of analytic derivs. for the equation-of-motion coupled
cluster (EOM-CC) method is reviewed. Special attention is paid to the
EOM-CC singles and doubles (EOM-CCSD) approxn., which has the same
computational scaling properties as the coupled-cluster singles doubles
(CCSD) ground state method and is therefore applicable to a wide range of
mol. systems. The detailed spin orbital equations that must be solved in
EOM-CCSD gradient calcns. are presented for the first time, and some
guidelines are discussed regarding their computational implementation.
Finally, use of the EOM-CCSD gradient method is illustrated by detg. the
structure, dipole moment components, harmonic frequencies and IR
intensities of formyl fluoride (HFCO) in its singlet excited (.eta., .pi.*)
state.

 
 

 

 

 

 
 

Accurate computations of 77Se NMR chemical shifts with the GIAO-CCSD
method. Buehl, Michael; Gauss, Juergen; Stanton, John F..
Organisch-Chemisches Institut, Universitaet Zuerich, Winterthurerstrasse
190, Zurich, Switz.

 

Chem. Phys. Lett. (1995), 241(3), 248-52.
CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
123:244930 AN 1995:717328 CAPLUS (Copyright 2002 ACS)

Abstract

Relative NMR chem. shifts (.delta.(Se)) were computed at the GIAO-MBPT(2)
and GIAO-CCSD levels for the selenium-contg. compds. H2Se, SeCO and CSe2.
With a large basis set, 77Se shifts calcd. at the GIAO-CCSD level for these
mols. are -335, -468 and 281 ppm, resp., relative to Me2Se. The results
for H2Se and CSe2 are in good agreement with gas phase NMR measurements of
-345 and 243 ppm, while the value for SeCO is close to a value measured in
soln. (-447 ppm). Correlation contributions to the shielding are severely
overestimated at the GIAO-MBPT(2) level; inclusion of higher-order effects
such as those included in GIAO-CCSD is essential for a quant. description
of selenium chem. shifts. The .delta.(Se) value of 2618 ppm predicted for
the transient H2C:Se at the GIAO-CCSD level indicates that the selenium
nucleus in this mol. is more de-shielded than in any other known compd.

 
 

 

 

 

 
 

Perturbative treatment of the similarity transformed Hamiltonian in
equation-of-motion coupled-cluster approximations. Stanton, John F.;
Gauss, Juergen. Inst. Theoretical Chemistry, Univ. Texas, Austin, TX,
USA.

 

J. Chem. Phys. (1995), 103(3), 1064-76. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 123:123390 AN
1995:694747 CAPLUS (Copyright 2002 ACS)

Abstract

A series of size-consistent approxns. to the equation-of-motion coupled
cluster method in the singles and doubles approxn. (EOM-CCSD) are developed
by subjecting the similarity transformed Hamiltonian .hivin.H = exp(-T)H
exp(T) to a perturbation expansion. Attention is directed to N and N - 1
electron final state realizations of the method defined by truncation of
.hivin.H at second order. Explicit spin-orbital equations for the energy
and its first deriv. are documented for both approaches [EOMEE-CCSD(2) and
EOMIP-CCSD(2), resp.], and have been implemented in a large-scale quantum
chem. program. Vertical ionization potentials calcd. by EOMIP-CCSD(2) are
shown to be equiv. to those of an approach presented recently by Nooijen
and Snijders [J. Chem. Phys. 102, 1681,(1995)]. Applications of both
EOMIP-CCSD(2) and EOMEE-CCSD(2) provide results for final state properties
that compare favorably with those obtained in full EOM-CCSD calcns. Anal.
of the computational aspects of the approx. and full EOM-CCSD methods shows
that the cost of EOMIP-CCSD(2) energy and gradient calcns. scales in
proportion to the fifth power of the basis set size, a significant savings
over the sixth power dependence of EOMIP-CCSD. This feature is of great
practical importance, as it shows that this N - 1 electron final state
approach has a large domain of applicability and is therefore likely to
become a valuable tool for application calcns. On the other hand, the same
cannot be said for EOMEE-CCSD(2) since its asymptotic computational
dependence and storage requirements are the same as the full EOMEE-CCSD
method.

 
 

 

 

 

 
 

Strong pseudo Jahn-Teller effect in the cyclic C3H radical. Stanton,
John F.. Institute for Theoretical Chemistry, Departments of Chemistry
and Biochemistry, The University of Texas at Austin, Austin, TX, USA

 

.
Chem. Phys. Lett. (1995), 237(1,2), 20-6. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 122:313907 AN
1995:539034 CAPLUS (Copyright 2002 ACS)

Abstract

A computational study of the structure and properties of the ground (~X
2B2) and first excited (~A 2A1) states of the cyclic C3H radical is
reported. Although spectroscopic analyses have been uniformly consistent
with a C2v structure for the electronic ground state, previous quantum
chem. investigations have predicted that in-plane distortion to a Cs
framework symmetry is an energetically favorable process. In contrast, the
present results predict a C2v equil. geometry for both the ground and
excited states. Problems related to symmetry breaking of the zeroth-order
wavefunction are avoided in this work by using an equation-of-motion
coupled cluster technique. Properties evaluated in calcns. for both the ~X
2B2 and ~A 2A1 states lend support to a published interpretation of the
microwave spectrum.

 

 

 

1994 publications

 

 

 

Analytical energy derivatives for ionized states described by the
equation-of-motion coupled cluster method. Stanton, John F.; Gauss,
Juergen. Inst. Theor. Chem., The University of Texas, Austin, Austin,
TX, USA
.

 

J. Chem. Phys. (1994), 101(10), 8938-44. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 122:115381 AN
1995:328161 CAPLUS (Copyright 2002 ACS)

Abstract

The theory for analytic energy derivs. of excited electronic states
described by the equation-of-motion coupled cluster (EOM-CC) method has
been generalized to treat cases in which ref. and final states differ in
the no. of electrons. While this work specializes to the sector of Fock
space that corresponds to ionization of the ref., the approach can be
trivially modified for electron attached final states. Unlike traditional
coupled cluster methods that are based on single determinant ref.
functions, several electronic configurations are treated in a balanced way
by EOM-CC. Therefore, this quantum chem. approach is appropriate for
problems that involve important nondynamic electron correlation effects.
Furthermore, a fully spin adapted treatment of doublet electronic states is
guaranteed when a spin restricted closed shell ref. state is used-a
desirable feature that is not easily achieved in std. coupled cluster
approaches. The efficient implementation of analytic gradients reported
here allows this variant of EOM-CC theory to be routinely applied to
multidimensional potential energy surfaces for the first time. Use of the
method is illustrated by an investigation of the formyloxyl radical (HCOO),
which suffers from notorious symmetry breaking effects.

 
 

 

 

 

 
   
   
 

On the equilibrium bond length of ammonia in the first excited singlet
state. Stanton, John F.; Kadagathur, Narayani S. Departments of
Chemistry and Biochemistry, University of Texas at Austin, Austin, TX,
USA.

 

J. Chem. Phys. (1995), 102(2), 1096-7. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 122:143058 AN
1995:301019 CAPLUS (Copyright 2002 ACS)

Abstract

The equation-of-motion coupled-cluster method in the singles and doubles
approxn.(EOM-CCSD) is used to evaluate an equil. N-H bond length of ammonia
in the first electronically excited singlet state.

 
 

 

 

 

 
 

Gauge-invariant calculation of nuclear magnetic shielding constants at the
coupled-cluster singles and doubles level. Gauss, Juergen; Stanton,
John F.. Institut Physikalische Chemie, Universitaet Karlsruhe,
Karlsruhe, Germany.

 

J. Chem. Phys. (1995), 102(1), 251-3. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 122:121368
AN 1995:284744 CAPLUS (Copyright 2002 ACS)

Abstract

The gauge-including AO (GIAO) method for the gauge-invariant calcn. of
nuclear magnetic shielding consts. was implemented at the coupled-cluster
singles and doubles (CCSD) level. A brief description of the theory and
its computational requirements is provided. Finally, the GIAO-CCSD method
is applied to calc. nuclear shielding consts. of N2O with two different
basis sets, the larger of which contains 153 contracted Gaussian functions.

 
 

 

 

 

 
 

Applications of post-Hartree-Fock methods: a tutorial. Bartlett, Rodney
J.; Stanton, John F.. Departments of Chemistry and Physics, University
of Florida, Gainesville, FL, USA.

 

Rev. Comput. Chem. (1994), 5
65-169. CODEN: RCCHEY ISSN: 1069-3599. Journal written in English.
CAN 122:9033 AN 1995:47056 CAPLUS (Copyright 2002 ACS)

Abstract

This article is a tutorial on the intelligent use of post-Hartree-Fock
(correlated) methods for the detn. of mol. structure and spectra.

 
 

 

 

 

 
 

C2H4B2N2: Ab Initio Prediction of Structure and Properties of Ring and
Chain Compounds. Cernusak, Ivan; Urban, Miroslav; Stanton, John F.;
Bartlett, Rodney J. Quantum Theory Project, University of Florida,
Gainesville, FL, USA.

 

J. Phys. Chem. (1994), 98(35), 8653-9.
CODEN: JPCHAX ISSN: 0022-3654. Journal written in English. CAN
122:10083 AN 1995:31071 CAPLUS (Copyright 2002 ACS)

Abstract

The authors present the MBPT(2) and coupled cluster description of the
structure, energetics, and vibrational spectra for three isomers of
1,4-diaza-2,5-diboracyclohexadiene. The ring and the acyclic Z- and
E-isomers of C2H4B2N2 (with hydrogens attached to the central CB bond in
"cis" or "trans" positions) as well as the transition state between the
ring and acyclic Z-isomer have been examd. All three mols. exhibit
remarkable thermodn. stability with respect to two cyanoborane monomers
(HCN-BH) and borazirene (HCNBH) and also with respect to the common H2BCN
mol. The authors demonstrate a necessity for using the coupled cluster
approach when reliable energy data are to be obtained. MBPT(2) is not
accurate enough. The cyclic isomer is the most stable species. The
barrier for the ring formation is acceptably low, suggesting that the
synthesis of this novel mol. is possible via its monomeric chain
precursors. Bonding anal. and the simulated IR spectra in the harmonic
approxn. based upon the MBPT(2) calcns. complete the information about
these mols., which may be interesting in connection with their potential
use in semiconductor technol.

 
 

 

 

 

 
 

Some predictions relevant to future spectroscopic observation of S1
vinylidene. Stanton, John F.; Gauss, Juergen. Inst. Theor. Chem.,
Univ. Texas, Austin, TX, USA.

 

J. Chem. Phys. (1994), 101(4),
3001-5. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 121:300361 AN 1994:700361 CAPLUS (Copyright 2002 ACS)

Abstract

The equation-of-motion coupled cluster method is used to obtain a precise
est. of the lowest adiabatic singlet-singlet excitation energy (S1 .rarw.
S0) of vinylidene. As photodetachment spectroscopy offers a promising
means for producing the first excited singlet state, harmonic vibrational
frequencies of S1 and the extent of mixing between the corresponding normal
modes and those of the anion are also calcd. To calibrate the calcns. and
provide a basis for empirical correction, parallel studies of the
previously obsd. T1 and T2 triplet states are reported. The adiabatic
excitation energy [S1 .rarw. S0] is estd. to be 3.12 .+-. 0.05 eV,
suggesting a photodetachment threshold energy of 3.61 .+-. 0.05 eV.
Progressions in the totally sym. C-C stretch and H-C-H bending modes should
be pronounced in the spectrum, with the extent of mode mixing similar to
that found for the S0 .rarw. anion and T1 .rarw. anion processes, but
differing in phase.

 
 

 

 

 

 
 

Stationary points on the S1 potential energy surface of C2H2. Stanton,
John F.; Huang, Chang Ming; Szalay, Peter G. Inst. Theor. Chem., Univ.
Texas, Austin, TX, USA.

 

J. Chem. Phys. (1994), 101(1), 356-65.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
121:133293 AN 1994:533293 CAPLUS (Copyright 2002 ACS)

Abstract

Recent theor. developments that facilitate characterization of
excited-state potential energy surfaces are applied to study five
stationary points on the S1 surface of C2H2. Although previous calcns.
have focused on the acetylenic trans and cis forms, the present research
predicts that the vinylidene isomer is the global min. on the potential
surface. However, a high activation barrier calcd. for rearrangement to
the trans isomer suggests that S1 vinylidene is not likely to be formed via
photoexcitation of ground-state acetylene. The trans and cis acetylenic
forms of the S1 state are shown to interconvert along an in-plane reaction
coordinate with an activation energy of .apprx.4000 cm-1, a value which is
significantly lower than usually assumed in spectroscopic analyses.

 
 

 

 

 

 
 

On the extent of spin contamination in open-shell coupled-cluster wave
functions. Stanton, John F.. Inst. Theor. Chem., Univ. Texas,
Austin, TX, USA.

 

J. Chem. Phys. (1994), 101(1), 371-4. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 121:118007
AN 1994:518007 CAPLUS (Copyright 2002 ACS)

Abstract

A study was made of the spin purity of states described by the
coupled-cluster singles and doubles (CCSD) approxn. based on unrestricted
and restricted open-shell Hartree-Fock (UHF and ROHF) determinants. By
contracting matrix elements of the S2 operator with reduced one- and
two-particle d. matrixes, the extent of spin contamination in ROHF-CCSD and
UHF-CCSD solns. was quant. characterized. Results are presented for a
representative set of mols. in doublet electronic states with UHF values of
S2 that range from 0.76 to 1.18. In these examples, ROHF-CCSD and UHF-CCSD
expectation values of S2 are similar, with both less than 0.015 above the
nominally exact value of 3/4. This finding illustrates the characteristic
insensitivity of CC methods with respect to the choice of orbitals.
Finally, the negligible differences between UHF-CCSD values of S2 and those
corresponding to a spin eigenfunction suggests that modest ref. state spin
contamination does not represent a serious problem when this method is
applied to chem. problems.

 
 

 

 

 

 
 

Analytic energy gradients for the equation-of-motion coupled-cluster
method: implementation and application to the HCN/HNC system. Stanton,
John F.; Gauss, Juergen. Inst. Theor. Chem., Univ. Texas, Austin, TX,
USA.

 

J. Chem. Phys. (1994), 100(6), 4695-8. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 121:92140 AN 1994:492140
CAPLUS (Copyright 2002 ACS)

Abstract

A recently developed theory for calcg. analytic first derivs. of the energy
for excited states treated by the equation-of-motion coupled-cluster
(EOM-CC) method has been implemented and applied to study potential energy
surfaces in the HCN/HNC system. The EOM-CC singles and doubles (EOM-CCSD)
approxn. is used to predict equil. structures, energies, dipole moments,
harmonic vibrational frequencies and IR intensities of HCN and HNC isomers
in both the the ~A1A" and ~B1A' excited states. In addn., the activated
complex for HCN .fwdarw. HNC isomerization in these states is investigated
theor. for the first time.

 
 

 

 

 

 
 

Calculation of C6 dispersion constants with coupled-cluster theory.
Stanton, John F.. Inst. Theor. Chem., Univ. Texas, Austin, TX, USA.

 


Phys. Rev. A (1994), 49(3), 1698-703. CODEN: PLRAAN ISSN: 0556-2791.
Journal written in English. CAN 120:227643 AN 1994:227643 CAPLUS
(Copyright 2002 ACS)

Abstract

The recently introduced coupled-cluster effective-hamiltonian (CC-EH)
approxn. was used to calc. the C6 dispersion consts. for selected interat.
potentials. The formal relation between C6 and an integral over the
dynamic polarizability evaluated at imaginary frequencies was used in the
calcns.; a discussion is given of its computational implementation within
the CC-EH framework. For the long-range interaction between two
ground-state beryllium atoms, the calcd. dispersion const. is in good
agreement with the full-configuration-interaction result. The
applicability of the method to both closed- and open-shell systems is
demonstrated by calcns. of C6 for interactions involving the ground states
of both 1S alk. earth metal and 2S alkali metal atoms. The results of
these calcns. are in acceptable agreement with recommended values for most
examples. However, significant discrepancies between the present ests. and
empirical upper and lower bounds are obsd. for all systems contg. calcium
atoms. This observation, as well as previous criticism of calcns. that
were used in establishing these bounds, suggests that a reexamn. of the
empirical ests. is warranted.

 

 

1993 publications

 

 

 

Many-body methods for excited state potential energy surfaces. I. General
theory of energy gradients for the equation-of-motion coupled-cluster
method. Stanton, John F.. Inst. Theor. Chem., Univ. Texas, Austin,
TX, USA.

 

J. Chem. Phys. (1993), 99(11), 8840-7. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 120:86757 AN
1994:86757 CAPLUS (Copyright 2002 ACS)

Abstract

The formal theory is presented for calcg. the analytic 1st deriv. of the
energy with respect to arbitrary perturbations within the
equation-of-motion coupled-cluster (EOM-CC) approxn. Through use of the
Dalgarno-Stewart interchange theorem (Z-vector method), terms involving
derivs. of the ground state cluster amplitudes are eliminated, leading to
the definition of a new quasiparticle de-excitation operator which
simplifies the theory and significantly reduces the expected cost assocd.
with studying potential energy surfaces for excited electronic states. For
both illustrative and pragmatic reasons, the final equations are cast in a
form similar to that developed for ground state CC energy derivs.,
involving contraction of effective 1- and 2-particle d. matrixes with
matrix elements of the differentiated Hamiltonian. Some aspects regarding
calcn. of the gradient are discussed with particular attention devoted to
similarities between the structure of the present formulas and those which
were previously implemented for the ground state problem.

 
 

 

 

 

 
 

Linear and cyclic isomers of the carbon tetraatomic molecules. A
theoretical study with coupled-cluster methods and large basis sets.
Watts, John D.; Gauss, Juergen; Stanton, John F.; Bartlett, Rodney J.
Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 

J. Chem. Phys.
(1992), 97(11), 8372-81. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 119:279305 AN 1993:679305 CAPLUS
(Copyright 2002 ACS)

Abstract

The ground electronic states of linear and rhombic C4 have been studied by
high level ab initio quantum chem. techniques. Geometries, harmonic
vibrational frequencies, IR intensities, and other quantities have been
detd. using 4s3p2d1f correlation consistent basis sets and coupled-cluster
methods including triple excitations. The linear-rhombic isomer energy
difference has been investigated with a range of basis sets, including a
5s4p3d2f1g correlation consistent set. The linear-rhombic energy
difference is influenced significantly by basis set, presence of triple
excitations, and the choice of ref. function for the open-shell linear
isomer. The effect of basis set variation is complex, but once a
reasonable quality of basis set has been achieved, further extensions favor
the rhombic isomer. The inclusion of triple excitations also favors the
rhombic isomer. The use of a RHF ref. function for the linear isomer
yields higher energies at the coupled-cluster level than if an UHF ref.
function is used, thereby again favoring the rhombic isomer. The most
complete calcns. of this study [coupled-cluster singles and doubles with
noniterative triples (CCSD(T)) with a 5s4p3d2f1g basis set] indicate that
the rhombic isomer is preferred by about 1 kcal mol-1. The coupled-cluster
vibrational frequencies of the linear isomer are all real, in agreement
with previous work, indicating that this isomer is not bent in the gas
phase. The IR intensities of linear C4 obtained in this work differ
significantly from those obtained previously with smaller basis sets and
either SCF theory or second-order perturbation theory. The present calcns.
give a dissocn. energy of C4 of 433 kcal mol-1, which is close to a
previous value obtained with the aid of an empirical correction, and
implies that several exptl. ests. of the heat of formation of C4 are
unreliable. Electron detachment energies of linear C4- and electron
affinities of C4 are computed with larger basis sets than previously and
are in very good agreement with recent anion photoelectron data.

 
 

 

 

 

 
 

A coupled-cluster based effective Hamiltonian method for dynamic electric
polarizabilities. Stanton, John F.; Barlett, Rodney J. Dep. Chem.,
Univ. Florida, Gainesville, FL, USA.

 

J. Chem. Phys. (1993), 99(7),
5178-83. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 119:279063 AN 1993:679063 CAPLUS (Copyright 2002 ACS)

Abstract

A coupled-cluster-based approach is described for calcg. dynamic
polarizabilities. In this procedure, the polarizability is calcd. by a
strategy that is formally equiv. to a sum over states corresponding to the
diagonal representation of a similarity-transformed Hamiltonian operator.
However, the explicit evaluation of excited-state wave functions and
energies is avoided. The present treatment is closely related to the
equation-of-motion coupled-cluster approxn. for excited states, and offers
an accurate approxn. to the second deriv. of the energy with respect to an
applied elec. field; the two approaches are equiv. in the limit that the
spectrum of states corresponding to the effective Hamiltonian is exact
within the basis set. Terms contributing to the second deriv., but
neglected in the proposed approach are shown to be insignificant for a
representative set of small mols. The method was applied to calc. the
polarizability of benzene at the wavelength of the sodium D line as well as
in the static field limit, by using a basis set of 192 contracted Gaussian
functions.

 
 

 

 

 

 
 

A CCSD(T) investigation of carbonyl oxide and dioxirane. Equilibrium
geometries, dipole moments, infrared spectra, heats of formation and
isomerization energies. Cremer, Dieter; Gauss, Juergen; Kraka, Elfi;
Stanton, John F.; Bartlett, Rodney J. Department of Theoretical
Chemistry, University of Goeteborg, Kemigarden 3, Goteborg, Swed.

 


Chem. Phys. Lett. (1993), 209(5-6), 547-56. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 119:202828 AN
1993:602828 CAPLUS (Copyright 2002 ACS)

Abstract

A CCSD and CCSD(T) investigation of carbonyl oxide (1) and its cyclic
isomer dioxirane (2) has been carried out employing DZ+P and TZ+2P basis
sets. Calcd. geometries, charge distributions, and dipole moments suggest
that 1 possesses more zwitterionic character [CCSD(T) dipole moment 4 D]
than has been predicted. 1 Can be distinguished from 2 by its IR spectrum
as indicated by CCSD(T) frequencies, intensities, and isotopic shifts. The
heats of formation .DELTA.H0f(298) for 1 and 2 are 30.2 and 6.0 kcal/mol,
resp.; the CCSD(T) barrier to isomerization from 1 to 2 is 19.2 kcal/mol.
Decompn. of 1 and 2 can lead to CO, CO2, H2O, H2 but not to free CH2, O2 or
O. Both isomers should be powerful epoxidn. agents in the presence of
alkenes, but they should differ in their ability to form cyclopropanes with
alkenes.

 
 

 

 

 

 
 

Does chlorine peroxide exhibit a strong, ultraviolet absorption near 250
nm? Stanton, John F.; Bartlett, Rodney J. Dep. Chem., Univ.
Florida, Gainesville, FL, USA.

 

J. Chem. Phys. (1993), 98(12),
9335-9. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 119:58721 AN 1993:458721 CAPLUS (Copyright 2002 ACS)

Abstract

A theor. investigation of the low-lying singlet electronic states of
chlorine peroxide (ClOOCl) is presented. Calcns. of excitation energies
and oscillator strengths at the equation of motion coupled-cluster singles
and doubles level predict that six excited states of ClOOCl are accessible
at photon wavelengths above 200 nm. These states occur in three sets of
pairs which can be approx. represented as symmetrized and antisymmetrized
excitations of electrons from lone pair and Cl-O bonding orbitals to
antibonding Cl-O orbitals. While small oscillator strengths are predicted
for transitions to the lowest two "pairs" of states, sizeable values are
predicted for the X1A .fwdarw. F1B transitions (0.02 and 0.08, resp.).
Furthermore, the estn. of the vertical excitation energy for these
processes is .apprxeq.5.20 eV, in satisfactory agreement with the exptl.
obsd. band max. of ClOOCl at 5.04 eV. The results of this work lend strong
support to the contention that photodissocn. of ClOOCl plays an important
role in the catalytic destruction of stratospheric ozone.

 
 

 

 

 

 
 

The equation-of-motion, coupled-cluster method. A systematic biorthogonal
approach to molecular excitation energies, transition probabilities, and
excited-state properties. Stanton, John F.; Bartlett, Rodney J. Dep.
Chem. Phys., Univ. Florida, Gainesville, FL, USA.

 

J. Chem. Phys.
(1993), 98(9), 7029-39. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 119:56391 AN 1993:456391 CAPLUS
(Copyright 2002 ACS)

Abstract

A comprehensive overview of the equation of motion coupled-cluster (EOM-CC)
method and its application to mol. systems is presented. By exploiting the
biorthogonal nature of the theory, it is shown that excited-state
properties and transition strengths can be evaluated via a generalized
expectation-value approach that incorporates both the bra and ket state
wave functions. Reduced d. matrixes defined by this procedure are given by
closed form expressions. For the root of the EOM-CC effective Hamiltonian
that corresponds to the ground state, the resulting equations are equiv. to
the usual expressions for normal single-ref. CC d. matrixes. Thus, the
method described in this paper provides a universal definition of
coupled-cluster d. matrixes, providing a link between EOM-CC and
traditional ground state CC theory. Excitation energy, oscillator
strength, and property calcns. are illustrated by means of several
numerical examples, including comparisons with full CI calcns. and a
detailed study of the 10 lowest electronically excited states of the cyclic
isomer of C4.

 

 

 

1992 publications

 

 

 

The ACES II program system. Stanton, John F.; Gauss, Juergen; Watts,
John D.; Lauderdale, Walter J.; Bartlett, Rodney J. Dep. Chem., Univ.
Florida, Gainesville, FL, USA.

 

Int. J. Quantum Chem., Quantum Chem.
Symp. (1992), 26(Proc. Int. Symp. At., Mol., Condens. Matter Theory
Comput. Methods, 1992), 879-94. CODEN: IJQSDI ISSN: 0161-3642. Journal
written in English. CAN 118:87769 AN 1993:87769 CAPLUS
(Copyright 2002 ACS)

Abstract

ACES II, a new program system for ab initio electronic structure calcns. is
described. The strengths of ACES II involve the use of many-body
perturbation theory (MBPT) and coupled-cluster (CC) theory for calcg. the
energy, geometry, spectra, and properties of small- to medium-sized mols.
This paper gives a brief overview of the ACES II project, describes many
features of the program system, and documents a no. of benchmark calcns.

 
 

 

 

 

 
 

Analytic restricted open-shell Hartree-Fock-many-body perturbation
theory(2) second derivatives. Gauss, Joergen; Stanton, John F.;
Bartlett, Rodney J. Inst. Phys. Chem., Univ. Karlsruhe, Karlsruhe,
Germany.

 

J. Chem. Phys. (1992), 97(10), 7825-8. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 118:45957 AN 1993:45957
CAPLUS (Copyright 2002 ACS)

Abstract

The communication extends the authors' previous work implementing the
restricted open-shell Hartree-Fock(ROHF)-MBPT anal. second derivs. to study
systems which exhibit considerable spin contamination. The energy,
geometry, vibrational frequencies, and IR intensities were calcd. for the
methylene imidogen (CH2N) for illustration.

 
 

 

 

 

 
 

Restricted-open-shell-Hartree-Fock-based many-body perturbation theory:
theory and application of energy and gradient calculations. Lauderdale,
Walter J.; Stanton, John F.; Gauss, Juergen; Watts, John D.; Bartlett,
Rodney J. Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 


J. Chem. Phys. (1992), 97(9), 6606-20. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 118:27633 AN 1993:27633 CAPLUS
(Copyright 2002 ACS)

Abstract

A new approach for many-body perturbation theory (MBPT) built upon a
restricted open-shell Hartree-Fock (ROHF) ref. function is presented.
ROHF-MBPT is shown to give much improved results compared to UHF (UHF) MBPT
in cases where there is large spin contamination of the UHF ref. function,
and to converge much more rapidly to the infinite-order coupled-cluster
result. Equations for anal. gradients at the MBPT(2) level are described
and implemented. ROHF-MBPT and restricted open-shell Hartree-Fock single-
and double-excitation coupled cluster (ROHF-CCSD) applications are
presented for several difficult cases. These include the structure and
electron affinity of the CN radical; structure, binding energy, and
vibrational frequencies of Li3; the structure and vibrational frequencies
for the unobserved FCS mol.; and the multiplet structure of the Ni atom.

 
 

 

 

 

 
 

On the choice of orbitals for symmetry breaking problems with application
to nitrogen oxide (NO3). Stanton, John F.; Gauss, Juergen; Bartlett,
Rodney J. Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 


J. Chem. Phys. (1992), 97(8), 5554-9. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 117:258453 AN 1992:658453 CAPLUS
(Copyright 2002 ACS)

Abstract

The suitability of using approx. Brueckner determinants in coupled-cluster
(CC) studies of orbital symmetry-breaking problems is explored. At least
for the cases considered here [NO3(2A2'), N2(2.SIGMA.g+) and NO2(2B2)], the
Brueckner ref. function, which by definition does not mix with single
excitations in the CC wave function, preserves symmetry exactly.
Furthermore, these solns. do not appear to be sensitive to the initial
guess, as the same sym. solns. are obtained starting with either
symmetry-broken or symmetry-constrained initial guesses. The same behavior
is not obsd. for Brueckner determinants obtained from CI calcns., where
symmetry-broken solns. are found. An evaluation of the merits of basing CC
calcns. on Brueckner determinants [B-CC] and quasirestricted Hartree-Fock
[QRHF] ref. functions (which also can be chosen to preserve symmetry
exactly) is presented. Calcns. carried out on the NO3 radical predict a
C2v equil. geometry at both the B-CC and QRHF-CC levels when triple
excitations are not included. However, all methods which include an
approx. treatment of triple excitation effects predict an extremely flat
potential surface which slightly favors the sym. [D3h] form of the mol.

 
 

 

 

 

 
 

Analytic restricted open-shell Hartree-Fock-many-body perturbation
theory(2) second derivatives. Gauss, Joergen; Stanton, John F.;
Bartlett, Rodney J. Inst. Phys. Chem., Univ. Karlsruhe, Karlsruhe,
Germany.

 

J. Chem. Phys. (1992), 97(10), 7825-8. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 118:45957 AN 1993:45957
CAPLUS (Copyright 2002 ACS)

Abstract

The communication extends the authors' previous work implementing the
restricted open-shell Hartree-Fock(ROHF)-MBPT anal. second derivs. to study
systems which exhibit considerable spin contamination. The energy,
geometry, vibrational frequencies, and IR intensities were calcd. for the
methylene imidogen (CH2N) for illustration.

 
 

 

 

 

 
 

Restricted-open-shell-Hartree-Fock-based many-body perturbation theory:
theory and application of energy and gradient calculations. Lauderdale,
Walter J.; Stanton, John F.; Gauss, Juergen; Watts, John D.; Bartlett,
Rodney J. Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 


J. Chem. Phys. (1992), 97(9), 6606-20. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 118:27633 AN 1993:27633 CAPLUS
(Copyright 2002 ACS)

Abstract

A new approach for many-body perturbation theory (MBPT) built upon a
restricted open-shell Hartree-Fock (ROHF) ref. function is presented.
ROHF-MBPT is shown to give much improved results compared to UHF (UHF) MBPT
in cases where there is large spin contamination of the UHF ref. function,
and to converge much more rapidly to the infinite-order coupled-cluster
result. Equations for anal. gradients at the MBPT(2) level are described
and implemented. ROHF-MBPT and restricted open-shell Hartree-Fock single-
and double-excitation coupled cluster (ROHF-CCSD) applications are
presented for several difficult cases. These include the structure and
electron affinity of the CN radical; structure, binding energy, and
vibrational frequencies of Li3; the structure and vibrational frequencies
for the unobserved FCS mol.; and the multiplet structure of the Ni atom.

 
 

 

 

 

 
 

On the choice of orbitals for symmetry breaking problems with application
to nitrogen oxide (NO3). Stanton, John F.; Gauss, Juergen; Bartlett,
Rodney J. Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 


J. Chem. Phys. (1992), 97(8), 5554-9. CODEN: JCPSA6 ISSN: 0021-9606.
Journal written in English. CAN 117:258453 AN 1992:658453 CAPLUS
(Copyright 2002 ACS)

Abstract

The suitability of using approx. Brueckner determinants in coupled-cluster
(CC) studies of orbital symmetry-breaking problems is explored. At least
for the cases considered here [NO3(2A2'), N2(2.SIGMA.g+) and NO2(2B2)], the
Brueckner ref. function, which by definition does not mix with single
excitations in the CC wave function, preserves symmetry exactly.
Furthermore, these solns. do not appear to be sensitive to the initial
guess, as the same sym. solns. are obtained starting with either
symmetry-broken or symmetry-constrained initial guesses. The same behavior
is not obsd. for Brueckner determinants obtained from CI calcns., where
symmetry-broken solns. are found. An evaluation of the merits of basing CC
calcns. on Brueckner determinants [B-CC] and quasirestricted Hartree-Fock
[QRHF] ref. functions (which also can be chosen to preserve symmetry
exactly) is presented. Calcns. carried out on the NO3 radical predict a
C2v equil. geometry at both the B-CC and QRHF-CC levels when triple
excitations are not included. However, all methods which include an
approx. treatment of triple excitation effects predict an extremely flat
potential surface which slightly favors the sym. [D3h] form of the mol.

 
 

 

 

 

 
 

Fock space multireference coupled-cluster theory for general single
determinant reference functions. Stanton, John F.; Bartlett, Rodney J.;
Rittby, C. Magnus L. Dep. Chem., Univ. Florida, Gainesville, FL,
USA.

 

J. Chem. Phys. (1992), 97(8), 5560-7. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 117:239998 AN
1992:639998 CAPLUS (Copyright 2002 ACS)

Abstract

The technique of Fock space multireference coupled-cluster (FSMRCC) theory
is applied for the first time to problems involving a high-spin open-shell
ground state. Explicit spin-orbital equations applicable to any single
determinant ref. state are presented and some computational aspects of
FSMRCC are discussed. The method is illustrated by two applications in
which calcns. are limited to single and double excitation operators
(FSMRCCSD). First, several basis sets and choices of open-shell ref.
function are used to calc. selected ionization potentials of O2. The
FSMRCCSD results obtained with a large generally contracted basis set are
uniformly within 0.1 eV of expt. In addn., FSMRCCSD is applied to a study
of symmetry breaking in the 3A2 state of CO2, a classic multireference
problem. The force const. for asym. distortion is shown to be predicted
correctly as pos., unlike ordinary single-ref. CCSD which predicts a
double-min. potential. The results of this paper suggest that the
open-shell ref. FSMRCC approach has wide applicability for the soln. of
chem. problems, particularly when significant nondynamic electron
correlation effects are present.

 
 

 

 

 

 
 

Interconversion of diborane(4) isomers. Stanton, John F.; Gauss,
Jurgen; Bartlett, Rodney J.; Helgaker, Trygve; Joergensen, Poul; Jensen,
Hans Jorgen A.; Taylor, Peter R. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

J. Chem. Phys. (1992), 97(2), 1211-16.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
117:119254 AN 1992:519254 CAPLUS (Copyright 2002 ACS)

Abstract

Highly correlated electronic structure calcns. using many-body perturbation
theory (MBPT) and coupled-cluster (CC) gradient techniques are applied to a
study of a reaction pathway which links the two forms [C2.nu. and D2d] of
B2H4. A reaction coordinate which preserves C2 symmetry is studied, as
this mechanism is allowed by orbital symmetry rules. However, calcns. show
that the min. energy path does not conform to this idealized mechanism.
Rather, the reaction coordinate bifurcates, and the transition state
contains no nontrivial elements of symmetry. At the level of partial
fourth-order many-body perturbation theory [SDQ-MBPT(4)] with a large
triple-zeta plus double polarization basis set, differences in distances
between the H atoms corresponding to the bridge atoms in the C2.nu. form
and the two B atoms [.delta.r(B-H) = |r(B1-H) - r(B2-H)-] are 0.14 and 0.81
.ANG., reflecting the pronounced asymmetry of the transition state
structure. The C2.nu. isomer should be the thermodynamically favored form
of B2H4. At the CC singles and doubles level with a noniterative treatment
of triple excitation effects, the barrier to isomerization is 6.3 kcal/mol
using a large generally contracted basis set. This result, along with
statistical arguments concerning the rate of reaction, suggest that equil.
may be established relatively rapidly, and that rigid mol. treatments of
B2H4 spectra may not be appropriate.

 
 

 

 

 

 
 

Investigation of the electronic structure and spectroscopy of Jahn-Teller
distorted fullerene ion(1+) C60+. Bendale, Rajiv D.; Stanton, John F.;
Zerner, Michael C. Quantum Theory Proj., Univ. Florida, Gainesville,
FL, USA.

 

Chem. Phys. Lett. (1992), 194(4-5-6), 467-71. CODEN:
CHPLBC ISSN: 0009-2614. Journal written in English. CAN 117:118879
AN 1992:518879 CAPLUS (Copyright 2002 ACS)

Abstract

The electronic structure and spectroscopy of C60+ is examd. using the INDO
model. The ground state of the ion distorts from Ih to either D5d, D3d or
D2h symmetry. The D5d structure is the equil. structure with a Jahn-Teller


stabilization energy of 8.1 kcal/mol; the D3d structure is related to a
cusp on the potential energy surface. Fifteen equiv. D2h structures
represent transition states for pseudorotation between the six equiv. D5d
structures, the pseudorotation barrier being 2.2 kcal/mol. The authors
predict the electronic spectrum of C60+ in D5d and Ih symmetry as well as
the ionization spectrum of C60, and compare these results with others.

 
 

 

 

 

 
 

Analytic evaluation of second derivatives using second-order many-body
perturbation theory and unrestricted Hartree-Fock reference functions.
Stanton, John F.; Gauss, Juergen; Bartlett, Rodney. Dep. Chem., Univ.
Florida, Gainesville, FL, USA.

 

Chem. Phys. Lett. (1992), 195(2-3),
194-9. CODEN: CHPLBC ISSN: 0009-2614. Journal written in English.
CAN 117:118693 AN 1992:518693 CAPLUS (Copyright 2002 ACS)

Abstract

Explicit spin-orbital equations are presented for the second deriv. of the
second-order many-body perturbation theory energy based on UHF ref.
functions, and their implementation is briefly discussed. Results are
reported for the elec. polarizabilities, harmonic vibrational frequencies
and IR intensities of the 3B1 state of CF2.

 
 

 

 

 

 
 

A systematic coupled-cluster investigation of structure and vibrational
frequencies of the lowest electronic states of ketenyl radical. Szalay,
Peter G.; Stanton, John F.; Bartlett, Rodney J. Quantum Theory Proj.,
Univ. Florida, Gainesville, FL, USA.

 

Chem. Phys. Lett. (1992),
193(6), 573-9. CODEN: CHPLBC ISSN: 0009-2614. Journal written in
English. CAN 117:47600 AN 1992:447600 CAPLUS (Copyright 2002
ACS)

Abstract

Structure and harmonic frequencies of ketenyl radical from UHF, MBPT(2),
CCSD and CCSD(T) calcns. using DZP and TZ2P basis sets are presented. The
equil. structure of the ground state is bent, while the linear structure
represents a saddle point on the potential energy surface. The results are
in good agreement with the available exptl. information. Geometrical
parameters and harmonic frequencies which are not available from expt. are
also presented.

 
 

 

 

 

 
 

Many-body perturbation theory with a restricted open-shell Hartree-Fock
reference. Lauderdale, Walter J.; Stanton, John F.; Gauss, Jurgen;
Watts, John D.; Bartlett, Rodney J. Quantum Theory Proj., Univ.
Florida, Gainesville, FL, USA.

 

Chem. Phys. Lett. (1991), 187(1-2),
21-8. CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
116:242052 AN 1992:242052 CAPLUS (Copyright 2002 ACS)

Abstract

A new, efficient ROHF-based MBPT method is presented. The method, which is
noniterative, invariant to transformations among occupied or virtual
orbitals, and generalizable to any order, is illustrated by application to
the UHF spin-contaminated CN radical and the H + OCH2 transition state.

 
 

 

 

 

 
 

Analytic MBPT energy derivatives: a powerful tool for the interpretation
and prediction of vibrational spectra for unusual molecules. Bartlett,
Rodney J.; Stanton, John F.; Watts, John D. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

Adv. Mol. Vib. Collision Dyn. (1991), 1B
139-67. CODEN: AMVDEY Journal; General Review written in English. CAN
116:138779 AN 1992:138779 CAPLUS (Copyright 2002 ACS)

Abstract

A review with 142 refs. on a no. of chem. applications of analytic energy
gradient evaluation at the level of second-order many-body perturbation
theory [MBPT(2)]. The emphasis is on studies carried out by the authors'
group on clusters of C and C-Si and electron-deficient boron hydrides and
related compds. The effects of electron correlation on theor. calcd.
harmonic force consts. and IR intensities are also discussed, as well as
the importance of basis set selection.

 
 

 

 

 

 
 

Stability and energetics of metastable molecules: tetraazatetrahedrane
(N4), hexaazabenzene (N6), and octaazacubane (N8). Lauderdale, Walter
J.; Stanton, John F.; Bartlett, Rodney J. Quantum Theory Proj., Univ.
Florida, Gainesville, FL, USA.

 

J. Phys. Chem. (1992), 96(3),
1173-8. CODEN: JPCHAX ISSN: 0022-3654. Journal written in English.
CAN 116:91813 AN 1992:91813 CAPLUS (Copyright 2002 ACS)

Abstract

By use of ab initio SCF (SCF), coupled-cluster, and many-body perturbation
theory (MBPT) methods, the potential nitrogen mols., tetrahedral N4 and
octahedral N8, are found to be metastable, corresponding to local min. on
their resp. potential energy surfaces. Barriers to unimol. dissocn. appear
to be large enough that the unknown mols. could be formed. Three addnl. N4
structures (C3, C2v, and D2h) are also found. Vibrational frequencies are
computed at the MBPT(2) level to aid in the identification of these unknown
mols. Similar anal. of the N6 analog of benzene demonstrates,
unambiguously, that even though hexaazabenzene is a local min. at the SCF
level, at the correlated level it is a second-order saddle point on the N6
surface.

 

 

 

1991 publications

 

 

 

Analytic evaluation of energy gradients at the coupled-cluster singles and
doubles level using quasirestricted Hartree-Fock open-shell reference
functions. Gauss, Jurgen; Stanton, John F.; Bartlett, Rodney J. Dep.
Chem., Univ. Florida, Gainesville, FL, USA.

 

J. Chem. Phys. (1991),
95(4), 2639-45. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 115:166814 AN 1991:566814 CAPLUS (Copyright 2002
ACS)

Abstract

Analytic formulas for the evaluation of energy gradients at the
coupled-cluster singles and doubles (CCSD) level for a special class of
open-shell quasi-RHF (QRHF) ref. functions are presented and their
computational implementation is discussed. The ref. functions considered
here are those which are formed from a closed-shell determinant for the
mol. in a different ionization state by either adding or removing a single
electron. Due to the approx. invariance of the CCSD model with respect to
the choice of orbitals, results obtained with QRHF-CCSD closely approx.
those from CCSD calcns. using UHF (UHF) or restricted open-shell
Hartree-Fock (ROHF) ref. functions in cases where the latter refs. are well
behaved. In cases where spin contamination (UHF) or symmetry breaking (UHF
and ROHF) effects are important, QRHF-CCSD offers an attractive alternative
since the resulting wave function is a projected eigenfunction of the spin
operator .cxa.S2 and transforms as a pure irreducible representation of the
mol. point group.

 
 

 

 

 

 
 

Coupled-cluster open-shell analytic gradients: implementation of the
direct product decomposition approach in energy gradient calculations.
Gauss, Juergen; Stanton, John F.; Bartlett, Rodney J. Dep. Chem., Univ.
Florida, Gainesville, FL, USA.

 

J. Chem. Phys. (1991), 95(4),
2623-38. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.

CAN 115:166813 AN 1991:566813 CAPLUS (Copyright 2002 ACS)

Abstract

Analytic energy gradients for the coupled-cluster singles and doubles
(CCSD) method have been implemented for closed-shell systems using RHF and
open-shell systems using UHF ref. functions. To achieve max. computational
efficiency, the basic theory has been reformulated in terms of
intermediates, thus reducing the no. of required floating-point operations,
and all computational steps are given in terms of matrix products in order
to exploit the vector capabilities of modern supercomputers. Furthermore,
the implementation has been designed to take full advantage of Abelian
symmetry operations. To illustrate the computational efficiency of our
implementation and in particular to demonstrate the possible savings due to
the exploitation of symmetry, computer timings and hardware requirements
are given for several representative chem. systems. In addn., the newly
developed analytic CCSD gradient methods are applied to calc. the equil.
geometry and energy splitting of the lowest singlet and triplet states of
the C4O2 mol.

 
 

 

 

 

 
 

Analytic energy gradients for open-shell coupled-cluster singles and
doubles (CCSD) calculations using restricted open-shell Hartree-Fock (ROHF)
reference functions. Gauss, Jurgen; Lauderdale, Walter J.; Stanton,
John F.; Watts, John D.; Bartlett, Rodney J. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

Chem. Phys. Lett. (1991), 182(3-4), 207-15.
CODEN: CHPLBC ISSN: 0009-2614. Journal written in English. CAN
115:142550 AN 1991:542550 CAPLUS (Copyright 2002 ACS)

Abstract

The theory of analytic energy gradients for the open-shell CCSD method
based on restricted open-shell Hartree-Fock (ROHF) ref. functions is
presented. The new CCSD gradient method is applied to the dissocn. of the
3A'' state of formaldehyde to H and HCO. Complete geometry optimization is
carried out for the initial reactant, the transition state of the reaction,
and for the dissocn. products. Noniterative triples appropriate to ROHF
are introduced and are used to define a new ROHF-CCSD(T) method. Using
this approach the zero-point corrections, the activation energy is calcd.
to be 21.0 kcal/mol and the exit barrier height is predicted to be 6.1
kcal/mol, both in excellent agreement with expt.

 
 

 

 

 

 
 

A benchmark coupled-cluster single, double, and triple excitation (CCSDT)
study of the structure and harmonic vibrational frequencies of the ozone
molecule. Watts, John D.; Stanton, John F.; Bartlett, Rodney J. Dep.
Chem., Univ. Florida, Gainesville, FL, USA.

 

Chem. Phys. Lett.
(1991), 178(5-6), 471-4. CODEN: CHPLBC ISSN: 0009-2614. Journal
written in English. CAN 114:235401 AN 1991:235401 CAPLUS
(Copyright 2002 ACS)

Abstract

We report a benchmark calcn. of the structure and harmonic vibrational
frequencies of the ozone mol. using the full coupled-cluster single,
double, and triple excitation model (CCSDT) with a DZP basis set. The
results offer a definitive ref. for assessing the accuracy of methods which
approx. the effects of the T3 cluster operator. For the bending and sym.
stretching frequencies, CCSD(T), CCSDT-2, and CCSDT-3 all give results
close to CCSDT. For the asym. stretching frequency, however, the values
predicted by the CCSD(T) and CCSDT-2 models are resp. about 100 cm-1 lower
and higher than the CCSDT value, while the CCSDT-3 value exceeds the CCSDT
value by 41 cm-1. It is suggested that effects of connected quadruple
excitations (T4) and higher cluster operators may have a significant effect
on the asym. stretching frequency.

 
 

 

 

 

 
 

Potential nonrigidity of the nitrate (NO3) free radical. Stanton, John
F.; Gauss, Jurgen; Bartlett, Rodney J. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

J. Chem. Phys. (1991), 94(5), 4084-7. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 114:235361
AN 1991:235361 CAPLUS (Copyright 2002 ACS)

Abstract

The form of the NO3 potential surface was inferred indirectly by calcg.
ionization potentials of the anion at different geometries with Fock space
multireference coupled-cluster theory, limited to single and double
excitations (FSMRCCSD). Harmonic frequencies calcd. at this level show the
D3h form of NO3 to be a local min. on the ESMRCCSD potential energy
surface.

 
 

 

 

 

 
 

A coupled-cluster study of the ground state of the carbon triatomic
monopositive ion. Watts, John D.; Stanton, John F.; Gauss, Jurgen;
Bartlett, Rodney J. Dep. Chem. Phys., Univ. Florida, Gainesville, FL,
USA.

 

J. Chem. Phys. (1991), 94(6), 4320-7. CODEN: JCPSA6 ISSN:
0021-9606. Journal written in English. CAN 114:215038 AN
1991:215038 CAPLUS (Copyright 2002 ACS)

Abstract

We report calcns. of the structure and harmonic frequencies of the 2B2
state of C3+, and the energy of this state relative to the linear geometry
(2.SIGMA.u+) using SCF and coupled-cluster (CC) methods, including the full
coupled-cluster single, double, and triple excitation (CCSDT) model. The
calcns. on 2B2 C3+ are the most complete treatment to date, and may be of
assistance in further exptl. detection of this species. The calcns. of the
2.SIGMA.u+ - 2B2 energy difference support a bent structure. These calcns.
also show that, compared with full CCSDT and CI single, double, triple, and
quadruple excitation (CISDTQ) results, CC methods which only approx.
include effects of connected triple excitations seem for this example to
give misleadingly small energy differences. A recent prediction by such
approx. methods that C3+ may be quasilinear is therefore questionable. In
the course of this work, certain practical difficulties in the SCF
description of 2B2 C3+ were encountered, namely symmetry breaking of the
restricted open-shell Hartree-Fock (ROHF) wave function, and the existence
of two distinct 2B2 UHF solns. We show that these can be alleviated by
using our quasirestricted Hartree-Fock CC approach (QRHF-CC).
Coupled-cluster single and double excitation (CCSD) calcns. based on a QRHF
ref. function consisting of neutral mol. orbitals are able to provide all
frequencies, unlike those based on a ROHF ref. function. This work shows
that QRHF-CC calcns. offer a convenient single ref. soln. to certain
problems involving symmetry breaking or other difficulties which
traditionally have been solved by multireference methods. This quality of
the QRHF-CC approach appears not to have been recognized previously.

 
 

 

 

 

 
 

A direct product decomposition approach for symmetry exploitation in
many-body methods. I. Energy calculations. Stanton, John F.; Gauss,
Jurgen; Watts, John D.; Bartlett, Rodney J. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

J. Chem. Phys. (1991), 94(6), 4334-45.
CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN
114:214713 AN 1991:214713 CAPLUS (Copyright 2002 ACS)

Abstract

An anal. of the matrix contractions involved in many-body perturbation
theory and coupled-cluster calcns. leads to a convenient strategy for
exploiting point group symmetry, by which the no. of floating point
operations can be reduced by as much as a factor of h2, where h is the
order of the mol. point group. Contrary to a statement in the literature,
the significant redn. in computation time realized in coupled-cluster
calcns. which exploit symmetry is not due to nonlinearities in the
equations. Rather, the savings of the fully vectorizable direct product
decompn. (DPD) method outlined here is assocd. with individual (linear)
contractions, and is therefore applicable to both linear and nonlinear
coupled-cluster models, as well as many-body perturbation theory. In addn.
to the large redn. in floating point operations made possible by exploiting
symmetry, core memory requirements are also reduced by a factor of
.apprxeq.h2. Implementation of the method for both open and closed shells
is reported. Computer timings and hardware requirements are given for
several representative chem. systems. Finally, the DPD method is applied
to the calcn. of the equil. geometry, totally sym. harmonic force field,
and vertical ionization potentials of the cubane mol. at the
coupled-cluster singles and doubles (CCSD) level.

 
 

 

 

 

 
 

The role of theory in studies of the diborane pyrolysis sequence.
Lipscomb, William N.; Stanton, John F.; Connick, W. B.; Magers, D. H.
Dep. Chem., Harvard Univ., Cambridge, MA, USA.

 

Pure Appl. Chem.
(1991), 63(3), 335-8. CODEN: PACHAS ISSN: 0033-4545. Journal written
in English. CAN 114:151400 AN 1991:151400 CAPLUS (Copyright 2002
ACS)

Abstract

Theor. studies to 4th order, or at coupled cluster levels using large basis
sets, have yielded a transition state for the reaction of BH3 with B2H6.
This state is .apprx.20 kcal/mol less stable than B3H9 (C3v) and 8 or 12
kcal/mol higher than H2 plus B3H7 of symmetry C2v or Cs, resp. Therefore,
most probably, B2H6 + BH3 yields B3H7 + H2 directly. Neither B3H9 (C3v)
nor BH5 trap BH3 in usual pyrolysis conditions, and B2H6 to B2H4 and H2 has
a higher barrier. A different BH3 adduct to B2H6, forming a 3-center bond
between B of BH3 and H (terminal) of B2H6, has .DELTA.Ee (electronic) of
-1.5 kcal/mol at the (531/31)MBPT(2) level.

 
 

 

 

 

 
 

Low-lying isomers of the chlorine oxide dimer: a theoretical study.
Stanton, John F.; Rittby, C. Magnus L.; Bartlett, Rodney J.; Toohey, Darin
W. Dep. Chem. Phys., Univ. Florida, Gainesville, FL, USA.


J. Phys. Chem. (1991), 95(6), 2107-10. CODEN: JPCHAX ISSN: 0022-3654.
Journal written in English. CAN 114:129429 AN 1991:129429 CAPLUS
(Copyright 2002 ACS)

Abstract

The ab initio thermochem. stabilities have been calcd. for three isomers of
the dimer of chlorine oxide (ClO) with many-body perturbation theory (MBPT)
and the infinite-order coupled-cluster (CC) approxn. Results indicate that
chlorine peroxide (ClOOCl) is the most stable form of Cl2O2, although the
alternative structure Cl(ClO2) cannot be definitively eliminated on the
basis of the present calcns. Calcns. of low-lying electronic excitation
energies for ClOOCl and Cl(ClO2) with Fock-space multireference CC suggest
that both isomers have transitions near 250 nm to energy levels that yield
either free chlorine atoms or considerably weakened ClO bonds. Addnl.
calcns. for the dipole moment and quadrupole moments of chlorine peroxide
are presented. In general, the results are in good agreement with a exptl.
studies which show that ClOOCl is the predominant form of Cl2O2 in the lab.
and that its photolysis in the UV region yields two chlorine atoms, thus
supporting theories of catalytic destruction of ozone by chlorine in the
winter polar stratosphere.

 
 

 

 

 

 
 

Point group symmetry and Cartesian force constant redundancy. Stanton,
John F.. Dep. Chem., Univ. Florida, Gainesville, FL, USA.

 

Int. J.
Quantum Chem. (1991), 39(1), 19-29. CODEN: IJQCB2 ISSN: 0020-7608.
Journal written in English. CAN 114:109208 AN 1991:109208 CAPLUS
(Copyright 2002 ACS)

Abstract

A procedure for identifying redundancy in the Cartesian force const. matrix
is described, and a prescription is given for generating the entire matrix
of second derivs. from the minimal set of information. A proof is supplied
which demonstrates that the no. of nonredundant rows corresponding to a
symmetry unique atom is necessarily less than three if the atom is
invariant with respect to a symmetry operation of the point group other
than the identity or inversion. Furthermore, only one row is required if
the atom lies on a threefold or higher-order rotation axis. An application
of the procedure to the evaluation of harmonic force consts. by numerical
differentiation of gradient vectors is briefly described.

 
 

 

 

 

 
 

Harmonic vibrational frequencies and infrared intensities from analytic
fourth-order many-body perturbation theory gradients. Stanton, John F.;
Watts, John D.; Bartlett, Rodney J. Dep. Chem., Univ. Florida,
Gainesville, FL, USA.

 

J. Chem. Phys. (1991), 94(1), 404-13. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 114:71428
AN 1991:71428 CAPLUS (Copyright 2002 ACS)

Abstract

Recently developed 4th-order many-body perturbation theory [MBPT(4)]
analytic gradient techniques were applied to a study of the harmonic
vibrational frequencies and IR intensities of prototype small polyat. mols.
Results are presented for the complete 4th-order model as well as for the
SDQ-MBPT(4) scheme, which neglects the contribution of triple excitations.
These results are also compared to frequencies and intensities obtained at
both higher and lower levels of theory. Differences between the 4th-order
results and those obtained with the infinite order coupled-cluster (CC)
counterparts of SDQ-MBPT(4) and MBPT(4) [CCSD and CC models including
triple excitations, the latter approximated here by CCSDT-1 and the UCC(4)
model] are negligible for all bending modes and stretches involving single
bonds. However, the infinite order effects included in CC models are
important for describing stretching modes of multiple bonds. Therefore,
iterative CC calcns. are needed to accurately characterize these regions of
the potential surface.

 

 

 

1989 publications

 

 

 

Highly correlated single reference studies of the ozone potential surface.
Dissociation and atomization energies. Stanton, John F.; Bartlett,
Rodney J.; Magers, David H.; Lipscomb, William N. Dep. Chem., Univ.
Florida, Gainesville, FL, USA.

 

Chem. Phys. Lett. (1989), 163(4-5),
333-8. CODEN: CHPLBC ISSN: 0009-2614. Journal written in English.

CAN 112:125558 AN 1990:125558 CAPLUS (Copyright 2002 ACS)

Abstract

Highly correlated many-body perturbation theory (MBPT) and coupled-cluster
(CC) calcns. based on a single determinant ref. function are applied to
computation of the dissocn. and atomization energies of ozone in its ground
electronic state. While results obtained in SCF calcns. are in error by >
100 kcal/mol for both quantities, many-body methods which include the
effects of triply substituted determinants (MBPT(4) and CCSDT-1) yield
values in relatively good agreement with expt. The present results are
superior to those obtained in multireference CI studies (MRCI). Comparable
accuracy is obtained with the empirically cor. G1 method and also in MRCI
calcns., provided a correction is applied to account for the effects of
unlinked clusters.

 
 

 

 

 

 
 

Electron correlation effects on the structure, stability and properties of
molecules. Stanton, John F.. Harvard Univ., Cambridge, MA, USA.

 

Avail. Univ. Microfilms Int., Order No. DA8914701. (1989), 181 pp.
From: Diss. Abstr. Int. B 1989, 50(5), 1965. Dissertation written in
English. CAN 112:125459 AN 1990:125459 CAPLUS (Copyright 2002
ACS)

 
 

 

 

 

 
 

An empirically adjusted Newton-Raphson algorithm for finding local minima
on molecular potential energy surfaces. Stanton, John F.; Bernholdt,
David E. Dep. Chem., Univ. Chicago, Chicago, IL, USA.

 

J. Comput.
Chem. (1990), 11(1), 58-63. CODEN: JCCHDD ISSN: 0192-8651. Journal
written in English. CAN 112:43064 AN 1990:43064 CAPLUS
(Copyright 2002 ACS)

Abstract

A simple extension of the Newton-Raphson method is proposed that approx.
accounts for anharmonicity in bond-stretching coordinates. By modeling
each bonded distance in a polyat. mol. as a Morse oscillator with no
anharmonic stretch-stretch or stretch-bend coupling, a multiplicative
correction factor to the Newton-Raphson step is derived. Representative
examples suggest that the rate of convergence of the proposed scheme is
typically faster than that of the std. Newton-Raphson method.

 
 

 

 

 

 
 

The equilibrium structure and harmonic vibrational frequencies of ozone:
coupled cluster results including triple excitations. Magers, David H.;
Lipscomb, William N.; Bartlett, Rodney J.; Stanton, John F.. Gibbs Chem.
Lab., Harvard Univ., Cambridge, MA, USA.

 

J. Chem. Phys. (1989),
91(3), 1945-7. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in
English. CAN 111:160731 AN 1989:560731 CAPLUS (Copyright 2002
ACS)

Abstract

The results obtained by the authors earlier (1989) are reinterpreted.

 
 

 

 

 

 
 

A theoretical study of the unimolecular dissociation of diborane.
Stanton, John F.; Bartlett, Rodney J.; Lipscomb, William N. Gibbs Chem.
Lab., Harvard Univ., Cambridge, MA, USA.

 

Editor(s): Maruani, Jean.
Mol. Phys., Chem., Biol. (1989), 3 357-63. Publisher: Kluwer,
Dordrecht, Neth CODEN: 56NAA2 Conference; General Review written in
English. CAN 111:121048 AN 1989:521048 CAPLUS (Copyright 2002
ACS)

Abstract

A review with 23 refs. is given with discussion of quantum chem. methods
used in study of the unimol. dissocn. of B2H6.

 
 

 

 

 

 
 

Early stages of diborane pyrolysis: a computational study. Stanton,
John F.; Lipscomb, William N.; Bartlett, Rodney J. Gibbs Chem. Lab.,
Harvard Univ., Cambridge, MA, USA.

 

J. Am. Chem. Soc. (1989),
111(14), 5165-73. CODEN: JACSAT ISSN: 0002-7863. Journal written in
English. CAN 111:29043 AN 1989:429043 CAPLUS (Copyright 2002
ACS)

Abstract

Methods of many-body perturbation theory and the coupled-cluster approxn.
were applied to a study of 2 elementary processes believed to play a role
in the pyrolysis of B2H6 BH3 + B2H6 .fwdarw. B3H9 (I) and B3H9 .fwdarw.


B3H7 + H2 (II). The reaction pathway for I proceeds through a transition
state stabilized by a donation-backdonation interaction reminiscent of that
involved in hydroboration and the activation and reaction enthalpies (at
400 K) for this process are .apprx.+14 and -5 kcal/mol, resp. Loss of H2
from the triborane(9) formed in I appears to occur with a negligible
kinetic barrier, leading to an isomer of B3H7 with C2v isomer. The theor.
reaction enthalpy for II of +9 kcal/mol more stable than the C2v isomer.
The theor. reaction enthalpy for II of +9 kcal/mol represents an approx.
upper limit to the activation energy for the most favorable route of H loss
from B3H9, suggersting that the barrier for this process is much lower than
has been assumed. This result, coupled casts doubt over the common belief
that step II is the slow step in the uncatalyzed pyrolysis of B2H6.
Relative rates for thermolysis of B2H6 and B2D6 are also computed from the
theor. energies, harmonic force fields, and structures. Regardless of
which step is assumed to be rate-detg., resulting values of kH/kD are
substantially smaller than the obsd. ratio of .apprx.5. Possible sources
of this discrepancy are discussed.

 
 

 

 

 

 
 

A theoretical investigation of the structure and properties of BH5.
Stanton, John F.; Lipscomb, William N.; Bartlett, Rodney J. Gibbs Chem.
Lab., Harvard Univ., Cambridge, MA, USA.

 

J. Am. Chem. Soc. (1989),
111(14), 5173-80. CODEN: JACSAT ISSN: 0002-7863. Journal written in
English. CAN 111:28835 AN 1989:428835 CAPLUS (Copyright 2002
ACS)

Abstract

The intermol. complex between borane (BH3) and H2 is studied with methods
based on many-body perturbation theory and the coupled-cluster approxn.
Calcns. with very large Gaussian basis sets indicate that BH5 is
particularly stable, with the min. of the intermol. potential roughly 6
kcal/mol below that of the sepd. monomers. A pronounced basis set
dependence is obsd.; highly correlated CCSD + T(CCSD) calcns. with an
unpolarized [3s2p]/[2s] basis set suggest a totally repulsive intermol.
interaction, while the singly polarized [3s2p1d]/[2s1p] basis predicts a
binding energy of 2.7 kcal/mol. Continued expansion of the basis is found
to systematically increase the stability. The structures of four isomers
are optimized at the MBPT(2)-[3s2p1d]/[2s1p] level and are compared with
results of previous theor. studies of BH5 and its org. analog, CH5+. Of
these isomers, only a Cs structure with the H2 subunit eclipsing one of the
B-H bonds of BH3 is a min.-another Cs and a C2v structure are transition
states for internal rotation and hydrogen scrambling, while a C4v isomer is
a second-order saddle point on the potential surface. MBPT(2) vibrational
frequencies and IR band intensities are also evaluated for the equil.
structure and are analyzed in terms of interacting mol. subunits. The
theor. enthalpic barriers for dissocn. and internal rearrangement at 298 K
are 2.4 and 6.6 kcal/mol, resp. Rate calcns. using a modified RRKM model
which permits an approx. inclusion of quantum effects suggest that proton
tunneling may play a significant role in the exptl. obsd. hydrogen
scrambling process. Overall, the results are qual. consistent with the
participation of BH5 in the aq. hydrolysis of tetrahydroborate anion.

 
 

 

 

 

 
 

Correlated studies of infrared intensities. Stanton, John F.; Lipscomb,
William N.; Magers, David H.; Bartlett, Rodney J. Gibbs Chem. Lab.,
Harvard Univ., Cambridge, MA, USA.

 

J. Chem. Phys. (1989), 90(6),
3241-9. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English.
CAN 110:181938 AN 1989:181938 CAPLUS (Copyright 2002 ACS)

Abstract

Coupled-cluster and many-body perturbation theories were applied to an
investigation of IR absorption intensities within the double-harmonic
approxn. In agreement with previous studies, both electron correlation and
basis set dependencies are significant, particularly for stretching
vibrations which involve H atoms. Intensities calcd. at the highly
correlated CCSD + T(CCSD) level with large Gaussian basis sets are in
reasonable agreement with expt. for HF and the .nu.2 and .nu.3 modes of
H2O, while the intensity of .nu.1 is overestd. even with a relatively large
53-CGTO basis. Intensities and harmonic frequencies calcd. at the SCF and
MBPT(2) levels with a double-.zeta. plus polarization (DZP) basis set are
presented for a no. of 1st row compds., and are compared to recent exptl.
values. Although agreement between exptl. and SCF intensities is poor,
these discrepancies are moderated considerably when correlation is
introduced at this simple level. The importance of including H
polarization functions in the basis set is discussed.

 
 

 

 

 

 
 

Highly correlated single-reference studies of the ozone potential surface.
I. Effects of high order excitations on the equilibrium structure and
harmonic force field of ozone. Stanton, John F.; Lipscomb, William N.;
Magers, David H.; Bartlett, Rodney J. Gibbs Chem. Lab., Harvard Univ.,
Cambridge, MA, USA.

 

J. Chem. Phys. (1989), 90(2), 1077-82. CODEN:
JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 110:141874
AN 1989:141874 CAPLUS (Copyright 2002 ACS)

Abstract

The potential energy surface of ozone in the vicinity of the equil.
geometry is investigated by single-ref. MBPT and CC methods. As expected,
the equil. structure and harmonic force field computed at the SCF level of
theory are in considerable error. When allowance is made for electron
correlation effects, the discrepancies between theory and expt. for the
equil. structure and totally sym. force field are significantly reduced,
and the results of different approxns. are in reasonable agreement with
accepted values. Asym. stretching frequencies, however, oscillate wildly
about the exptl. value suggesting that the Cs subspace of the potential
surface poses a great challenge to theory. A possible cause of this
behavior is discussed.

 
 

 

 

 

 
 

Electron correlation effects on the ground-state structure and stability of
triborane(9). Stanton, John F.; Lipscomb, William N.; Bartlett, Rodney
J.; McKee, Michael L. Gibbs Chem. Lab., Harvard Univ., Cambridge, MA,
USA.

 

Inorg. Chem. (1989), 28(1), 109-11. CODEN: INOCAJ ISSN:
0020-1669. Journal written in English. CAN 110:45117 AN 1989:45117
CAPLUS (Copyright 2002 ACS)

Abstract

Coupled cluster and MBPT theories are applied to a study of the
ground-state structure and thermodn. stability of triborane(9). Electron
correlation effects have a dramatic influence on the predicted equil.
structure of B3H9, shortening the nominally nonbonded
B.cntdot..cntdot..cntdot.B distances by 0915 .ANG. relative to the
min.-energy self-consistent-field structure. Reaction energies for B2H6 +
BH3 .fwdarw. B3H9 are also sensitive to correlation effects, which
stabilize the product by approx. 20 kcal/mol. For a [4s3p1d]/[3s1p]
Gaussian basis set, the MBPT(4) electronic reaction energy is -7.9
kcal/mol. Introduction of thermal and entropic effects, however, favors
the reactants. At room temp., the Gibbs free energy change for the
reaction is predicted to be +4.6 kcal/mol, which suggests that B3H9 does
not serve to trap BH3 during the pyrolysis of diborane.

 

 

 

1988 publications

 

 

 

Computation of quadratic electric dipole moment functions. Stanton,
John F.; Magers, David H. Gibbs Chem. Lab., Harvard Univ., Cambridge,
MA, USA.

 

J. Chem. Phys. (1988), 88(12), 7650-2. CODEN: JCPSA6
ISSN: 0021-9606. Journal written in English. CAN 109:197356 AN
1988:597356 CAPLUS (Copyright 2002 ACS)

Abstract

A numerically reliable method is described for calcn. of mol.
elec.-dipole-moment functions. This scheme, which is an extension of a
method due to A. Komornicki and J. W. McIver (1979), was applied to
computation of the quadratic dipole-moment function of water using
many-body perturbation theory. A pronounced basis-set dependence was obsd.
for the stretch and stretch-stretch coeffs. of the quadratic expansion of
the dipole moment; whereas, those involving the bending symmetry coordinate
appeared to be less sensitive. Previously published ab-initio
dipole-moment functions are discussed, and compared to the present results.

 
 

 

 

 

 
 

Structure, energetics, and vibrational spectra of beryllium borohydride
isomers. Stanton, John F.; Lipscomb, William N.; Bartlett, Rodney J.
Gibbs Chem. Lab., Harvard Univ., Cambridge, MA, USA.

 

J. Chem. Phys.
(1988), 88(9), 5726-34. CODEN: JCPSA6 ISSN: 0021-9606. Journal
written in English. CAN 109:80110 AN 1988:480110 CAPLUS
(Copyright 2002 ACS)

Abstract

Several structures of beryllium borohydride (BeB2H8) have been studied with
techniques based on many-body perturbation theory (MBPT). Energies calcd.
at the MBPT(4) level with a basis set of 85 contracted Gaussian functions
support the two structure hypothesis of Nibler, although a qual. MBPT(4)
asym. stretching potential indicates that the C3v double-min. structure is
unstable with respect to a more highly sym. D3d model with two sets of
equiv. triple hydrogen bridges. The theor. free energy difference between
the D3d structure and a diborane-like model having D2d symmetry is 1.1
kcal/mol, with the latter predicted to be more stable. Uncertainties in
the theor. force field and those resulting from the finite nature of the
basis set, however, may change this value by several kcal/mol. Principal
evidence for the two-structure proposal comes from MBPT(2) harmonic
frequencies and IR absorption intensities, which are found to be consistent
with the published vibrational spectra. These are used in a tentative
assignment of most of the obsd. features. The exptl. obsd. dipole moment
of 2.06 D cannot be explained at present.

 

 

 

1987 publications
 

A coupled-cluster and MBPT study of diborane(6) and borane(3). Stanton,
John F.; Bartlett, Rodney J.; Lipscomb, William N. Gibbs Chem. Lab.,
Harvard Univ., Cambridge, MA, USA.

 

Chem. Phys. Lett. (1987),
138(6), 525-30. CODEN: CHPLBC ISSN: 0009-2614. Journal written in
English. CAN 107:141565 AN 1987:541565 CAPLUS (Copyright 2002
ACS)

Abstract

The CC and MBPT are applied to study of diborane and borane. Results for
the dissocn. enthalpy of the former calcd. with two large contracted
Gaussian basis sets at the CCSD + T(CCSD) level of approxn. are in
excellent agreement with expt. In addn., harmonic vibrational frequencies
and IR absorption intensities computed at the correlated level are used in
a crit. assessment of the currently accepted vibrational assignments for
both species.

 

 

 

1986 publications

 

 

 

Application of Miller's classical path approximation to the calculation of
the shape of one-dimensional anharmonic probability distributions.
Stanton, John F.. Gibbs Chem. Lab., Harvard Univ., Cambridge, MA,
USA.

 

Chem. Phys. Lett. (1986), 132(3), 311-14. CODEN: CHPLBC ISSN:
0009-2614. Journal written in English. CAN 106:90329 AN 1987:90329
CAPLUS (Copyright 2002 ACS)

Abstract

Miller's classical-limit expression for the thermally averaged probability
distribution of an ensemble of 1-dimensional oscillators was applied to a
model anharmonic system, the Morse oscillator. This method, which requires
no knowledge of the stationary state wavefunctions or eigenvalues,
accurately characterizes the shape of the distribution function at very low
as well as higher temps.

 
 

 

 

 

 
 

Simple extension of the classical path treatment for anharmonic
oscillators. Stanton, John F.; Bartell, Lawrence S. Dep. Chem.,
Harvard Univ., Cambridge, MA, USA.

 

J. Phys. Chem. (1986), 90(21),
4975-9. CODEN: JPCHAX ISSN: 0022-3654. Journal written in English.
CAN 105:158977 AN 1986:558977 CAPLUS (Copyright 2002 ACS)

Abstract

W. H. Miller's (1971) classical path approxn. for calcg. the probability d.
of a thermal ensemble of oscillators was modified to incorporate
anharmonicity more fully in the effective potential energy. Although this
extension is based on a simple attempt to avoid integrals more complex than
those already treated by Miller, it correctly characterizes the asymmetry
of the thermal distribution of Morse-like oscillators at temps. twice as
low as those of Miller's formulation and 8 times lower than those of a
classical Boltzmann distribution. Various levels of approxn. are discussed
briefly along with implications in exptl. mol. structure analyses.

 

 

 

1984 publications

 

 

 

Electron diffraction studies of hot molecules. IV. Asymmetries of
nonbonded distribution functions of sulfur hexafluoride, tetrafluorosilane
and carbon tetrachloride. Bartell, Lawrence S.; Stanton, John F..
Dep. Chem., Univ. Michigan, Ann Arbor, MI, USA.

 

J. Chem. Phys.
(1984), 81(9), 3792-5. CODEN: JCPSA6 ISSN: 0021-9606. Journal written
in English. CAN 102:14530 AN 1985:14530 CAPLUS (Copyright 2002
ACS)

Abstract

Highly significant improvements in the agreement between obsd. and calcd.
intensities of electrons diffracted by hot mols. were obtained by
optimizing the skew parameter .cxa.a for the nonbonded distributions.
Derived .cxa.a values (.+-. 2.sigma.) were 2.75(11), 0.72(13), and 2.0(4)
.ANG.-1 for SF6, SiF4, and CF4, resp. These measured skew parameters are
.apprx.2 .ANG.-1 higher in each case than values previously proposed on the
basis of Morse asymmetry factors and the nonlinear transformation between
curvilinear and normal coordinates of mols. The principal factors
responsible for the increase is the previously unknown intrinsic
anharmonicity in bending deformations. SiF4 has a lower .cxa.a value than
the other mols. studied primarily because its bending force const. is
lower, relative to stretching. Practical as well as theor. implications of
present findings are discussed.