APPLICATION OF DENSITY FUNCTIONAL THEORY TO INFRARED-ABSORPTION INTENSITY CALCULATIONS ON MAIN GROUP MOLECULES

Approximate density functional theory has been evaluated as a practical tool for calculations on infrared vibrational frequencies and absorption intensities. The density functional schemes included the local density approximation (LDA) by Gunnarson [Phys. Rev. B 10, 1319 (1974)] as well as a self-consistent nonlocal density functional method (LDA/NL) in which the gradient corrected exchange term by Becke [Phys. Rev. A 38, 3098 ( 1988) ] and the gradient corrected correlation term by Perdew [Phys. Rev. B 33, 8822 (1986) ] has been added to LDA. The LDA and LDA/NL schemes have been applied to calculations on the infrared vibrational frequencies and absorption intensities of H2O, NH3, H, CO, C, H4, CH3 OH and oxirane. The calculations were carried out with two basis sets of respectively double-zeta plus polarization (DZP) and triple-zeta plus polarization (TZP) quality. The study has demonstrated that vibrational frequencies are relatively insensitive to the level of density functional theory as well as to the choice of basis set. The calculated frequencies are, in general, in better agreement with experiment than values obtained by ab initio Hartree-Fock calculations. Large basis sets of TZP quality are, on the other hand, required for intensities. It is further shown that a small but consistent improvement in the calculated ir intensities is obtained by including nonlocal corrections. The LDA/NL method with a TZP basis set affords ir intensities and frequencies of the same quality as post-Hartree-Fock methods with an average deviation in the intensities of 20%-40%.