NATURAL POLYELECTRON POPULATION ANALYSIS

A method to perform a polyelectron population analysis of correlated m olecular orbital wave functions on the basis of natural atomic orbital s (NAOs), as given by Weinhold, is presented. The method allows calcul ations of the probabilities of finding various types of electronic eve nts occurring in some target AO positions, including the contributions of ionic and covalent resonance structures. This method is general an d neither the theory nor the developed algorithm limit the number of e lectrons and holes that can be considered. Thus, the analyzed MO wave function can be a usual CI or a MCSCF one, and apart from Weinhold's N AOs, any other type of orthogonal AOs can be used as analyzers, provid ed that these AOs are linear combinations of the SCF-AOs. Numerical ap plications are given for ethylene, formaldehyde, butadiene, and acrole ine, by adopting various AO basis-set levels (STO-4G, 4-31G, and 6-31G ) and by analyzing correlated wave functions (CISD). Improvements in the polyelectron populations when increasing the quality of AO basis s ets and the corresponding valence NAOs are revealed by several example s. Furthermore, it is shown that the electronegativity of oxygen in ac roleine only has an effect on contributions of ionic and covalent reso nance structures, but not on delocalization of the double bonds. (C) 1 994 John Wiley & Sons, Inc.