LOCAL DENSITY FUNCTIONAL THEORY CALCULATION OF THE INPLANE FORCE-FIELD AND VIBRATIONAL FREQUENCIES OF CONJUGATED MOLECULES - BENZENE AND OCTATETRAENE

The in-plane force field and the vibrational frequencies of benzene and octatetraene are calculated by DGauss, the local density functional theory (LDFT) program available from Cray Research. Default procedures are adopted for the energy functional, basis set (DZVP), integral precision and convergence requirements. The calculated vibrational frequencies are in much better agreement with the experiment than the raw ab initio data obtained at Hartree-Fock self consistent field (HF SCF) level with comparable basis sets. Comparison of the force constants obtained by LDFT and HF SCF theory shows that these two levels of theory are complementary and vibrational spectroscopy should benefit from their combined use.