EVALUATION OF VIBRATIONAL FORCE-FIELDS DERIVED BY USING SEMIEMPIRICAL AND ABINITIO METHODS

Modern semiempirical computational methods were surveyed for their ability to predict vibrational force fields. The MNDO, AM1, and PM3 methods were used to evaluate the harmonic force constants and vibrational frequencies of a series of structurally related molecules. Full symmetry and symmetrized internal valence force constants are reported. Results are compared to the experimental harmonic force constants and to values obtained from minimal and extended basis ab initio calculations. The PM3 Hamiltonian is evaluated for the first time and predicts frequencies with greater accuracy than MNDO or AM1. The semiempirical results agree well with ab initio values.