Systematic ab initio gradient calculation of molecular geometries, force constants, and dipole moment derivatives

Abstract

Systematic ab initio gradient calculation of molecular geometries, force constants, and dipole moment derivatives is described. A new basis set, denoted 4-21, is presented for first-row atoms. It is nearly equivalent to the 4-31G set but requires less computational effort. Completely optimized Hartree-Fock geometries of 18 molecules are compared using several basis sets, with and without polarization functions. The question of the best representation of molecular force fields is discussed, and a set of standardized internal coordinates is suggested for future work. Quadratic and the most important cubic force constants and dipole moment derivatives of first-row hydrides are calculated using the 4-21 basis set, and the results are compared with those from other basis sets, including near-Hartree-Fock ones. Force-field calculations on larger molecules with the 4-21 basis are summarized. A general formulation of the rotational correction to dipole moment derivatives is given.