AB-INITIO STUDY OF S-TRANS-1,3-BUTADIENE USING VARIOUS LEVELS OF BASIS-SET AND ELECTRON CORRELATION - FORCE-CONSTANTS AND EXPONENTIALIY SCALED VIBRATIONAL FREQUENCIES

The structure, vibrational spectra, and force constants of s-trans-1,3 -butadiene have been studied with ab initio Hartree-Fock (HF), Moller- Plesset second-order perturbation (MP2), and Moller-Plesset fourth ord er perturbation (MP4) calculations using various basis sets. The vibra tional frequencies show a very sensitive dependence on basis sets and electron correlation. The root mean square (rms) frequency deviation b etween experiment and theories of MP2/6-311G(2d,p) and MP4/6-31G is 2 2 cm(-1). The scaled frequencies with exponentially varying weight agr ees better with experiment than those with constant weight, because hi gher frequencies tend to be more overestimated. The exponentially scal ed rms frequency deviation by MP2/6-311G(2d,p) reduces down to 12 cm(- 1), which is in surprisingly good agreement with experiment. The error due to the incompleteness of basis sets is found to be more important at the MP2 level than the error due to the electron correlation. The backbone stretching force constants are in excellent agreement with th e experimental values. The C-C/C=C force constant is not so large as a nticipated for the construction of reliable polyacetylene force fields . Otherwise, the force constant in polyacetylene may be different from that in butadiene.