A method of calculating electron correlations for large molecules involving C, N, and H atoms

Abstract

A way of calculating the electron correlation energy for the ground state of large organic molecules is presented. It is demonstrated that various contributions to the correlation energy may be described by simple analytic expressions. In the case of interatomic correlations, they depend only on the bond length, its kind (e.g., σ or π bond) and the atoms involved in the bonding. Intraatomic correlations for a given atom are shown to be determined by its total charge and the fraction of p electrons. The method is developed by starting from semiempirical (self-consistent field) calculations and building correlations into it. It is straightforward and simple to apply. Moreover, it provides considerable physical insight into the phenomenon of electron correlations. A number of tests of its accuracy are presented by considering small molecules for which a comparison with other calculations can be made. An overall satisfactory agreement is found.