DENSITY-FUNCTIONAL THEORY STUDY OF VIBRATIONAL-SPECTRA .1. PERFORMANCE OF SEVERAL DENSITY-FUNCTIONAL METHODS IN PREDICTING VIBRATIONAL FREQUENCIES

Harmonic vibrational frequencies of several small organic molecules wh ich were used to validate the scaled quantum mechanical (SQM) force fi eld procedure of Pulay et al. were calculated using six popular densit y functional (DFT) methods and compared with experimental results. The combination of Becke's exchange with either Lee-Yang-Parr (BLYP) or P erdew's correlation functionals (BP86) reproduces the observed frequen cies satisfactorily with deviations similar to those of the Hartree-Fo ck SQM methods. Three hybrid DFT methods are found to yield frequencie s which were generally higher than the observed fundamental frequencie s. When the calculated frequencies were compared with 'experimental' h armonic frequencies however, Becke's three-parameter hybrid method wit h Lee-Yang-Parr correlation functional is found to be slightly more ac curate, especially for C-H stretching modes. The results indicate that BLYP calculation is a very promising approach for understanding the o bserved spectral features.