STUDY OF CH4, C2H2, C2H4, AND C6H6 USING KOHN-SHAM THEORY

Self-consistent Kohn-Sham (KS) theory with local density approximation (LDA) and gradient-corrected exchange-correlation (BLYP) functionals are used to optimize the geometries and calculate the frequencies of methane, acetylene, ethylene, and benzene. Large basis sets are employed with accurate numerical quadrature. The predictions are compared with ab initio calculations and with experiment. KS (BLYP) bond lengths are a little long with the result that the stretching frequencies are too small, but good predictions are obtained for bending frequencies. In particular, the omega14 B2u mode of benzene is predicted to within 10 cm-1, unlike all of the ab initio methods which apparently require multiconfiguration methodology. There is a need to improve functionals so that a better description of stretching frequencies is obtained.