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Please use this identifier to cite or link to this item: http://hdl.handle.net/1942/3585

Title: AN ACCURATE AB-INITIO QUARTIC FORCE-FIELD AND VIBRATIONAL FREQUENCIES FOR CH4 AND ISOTOPOMERS
Authors: LEE, TJ
MARTIN, Jan
TAYLOR, PR
Issue Date: 1995
Publisher: AMER INST PHYSICS
Citation: JOURNAL OF CHEMICAL PHYSICS, 102(1). p. 254-261
Abstract: A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm−1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 re by 0.0015 Å. Our best estimate for re is 1.0862±0.0005 Å.
Notes: LIMBURGS UNIV CENTRUM,DEPT SBG,B-3590 DIEPENBEEK,BELGIUM. UNIV INSTELLING ANTWERP,DEPT CHEM,B-2610 WILRIJK,BELGIUM. SAN DIEGO SUPERCOMP CTR,SAN DIEGO,CA 92186.LEE, TJ, NASA,AMES RES CTR,MOFFETT FIELD,CA 94035.
URI: http://hdl.handle.net/1942/3585
DOI: 10.1063/1.469398
ISI #: A1995PZ71400027
ISSN: 0021-9606
Type: Journal Contribution
Appears in Collections: Research publications

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