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 m ethane, acetylene, ethylene, and benzene. Large basis sets are employe d with accurate numerical quadrature. The predictions are compared wit h ab initio calculations and with experiment. KS (BLYP) bond lengths a re a little long with the result that the stretching frequencies are t oo small, but good predictions are obtained for bending frequencies. I n particular, the omega14 B2u mode of benzene is predicted to within 1 0 cm-1, unlike all of the ab initio methods which apparently require m ulticonfiguration methodology. There is a need to improve functionals so that a better description of stretching frequencies is obtained.