Atomic electron populations by molecular orbital theory

Abstract

The detailed distribution of electrons in molecules can be broken down into atomic orbital populations if an atomic orbital basis is used for a molecular orbital wave function (LCAO). A total assignment of all electrons in a molecule to atomic orbitals (and hence to particular atoms) can be made by finding gross populations as suggested by Mulliken. These are of some interest in discussing various molecular properties associated with charge density, particularly if a minimal basis is used consisting of only atomic orbitals in the valence and inner shells. It has now become possible to evaluate atomic orbital populations for a range of molecules both by semi-empirical and ab initio methods. The semi-empirical methods (based on zero differential overlap) utilize some experimental atomic data, but can be applied to large systems with little difficulty. Ab initio methods generally require the evaluation of a large number of difficult integrals, but they are becoming available for many small systems. The main aim of this paper is to compare atomic electron populations obtained by the semi-empirical INDO molecular orbital theory with some values from full SCF calculations using a minimal basis set of exponential-type functions. Agreement is moderately good, but only if the exponents in the full calculations are chosen carefully to minimize the total energy.