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An Accurate ab initio Quartic Force Field and Vibrational Frequencies for CH4 and IsotopomersA 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(exp -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 r(e), by 0.0015 A. Our best estimate for r, is 1.0862 +/- 0.0005 A.
Document ID
19970017577
Acquisition Source
Ames Research Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Lee, Timothy J.
(NASA Ames Research Center Moffett Field, CA United States)
Martin, Jan M. L.
(Limburgs Univ. Diepenbeek, Belgium)
Taylor, Peter R.
(San Diego Supercomputer Center San Diego, CA United States)
Date Acquired
August 17, 2013
Publication Date
January 1, 1995
Publication Information
Publication: Journal of Chemical Physics
Volume: 102
Issue: 1
ISSN: 0021-9606
Subject Category
Inorganic And Physical Chemistry
Report/Patent Number
NAS 1.15:11968
NASA-TM-111968
Accession Number
97N71672
Funding Number(s)
CONTRACT_GRANT: NSF DASC-89-02825
Distribution Limits
Public
Copyright
Public Use Permitted.
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