Polarized Molecular Orbital Model Chemistry. I. Ab Initio Foundations.

The objective of this paper is to examine the minimal requirements for obtaining semiquantitative polarizabilities of molecules, in order to provide a well-founded starting point for a new semiempirical molecular orbital formulation that is more suitable than presently available methods for simulating electronic polarization effects. For this purpose, we present polarizability calculations for 38 molecules with 36 basis sets, including many unconventional ones, and five semiempirical molecular orbital theories based on neglect of diatomic differential overlap. We conclude that two basis sets are particularly promising to serve as bases for semiempirical improvement, namely STO-3G(,P), in which diffuse p functions are added to all hydrogens, and 3-(21,3,21)G in which a minimal basis set is augmented with one extra s function on every atom. We especially recommend the former because all intra-atomic overlap integrals are zero by symmetry, which makes it a better candidate for neglect-of-differential-overlap treatments.