POLARIZED ATOMIC ORBITALS FOR SELF-CONSISTENT-FIELD ELECTRONIC-STRUCTURE CALCULATIONS

We present a new self-consistent field approach which, given a large ' 'secondary'' basis set of atomic orbitals, variationally optimizes mol ecular orbitals in terms of a small ''primary'' basis set of distorted atomic orbitals, which are simultaneously optimized. If the primary b asis is taken as a minimal basis, the resulting functions are termed p olarized atomic orbitals (PAO's) because they are valence (or core) at omic orbitals which have distorted or polarized in an optimal way for their molecular environment. The PAO's derive their flexibility from t he fact that they are formed from atom-centered linear-combinations of the larger st of secondary atomic orbitals. The variational condition s satisfied by PAO's are defined, and an iterative method for performi ng a PAO-SCF calculation is introduced. We compare the :PAO-SCF approa ch against full SCF calculations for the energies, dipoles, and molecu lar geometries of various molecules. The PAO's are potentially useful for studying large systems that are currently intractable with larger than minimal basis sets, as well as offering potential interpretative benefits relative to calculations in extended basis sets. (C) 1997 Ame rican Institute of Physics.