A non-orthogonal pair theory of molecular electronic structure

Abstract

An unrestricted pair theory, for N singlet coupled pairs, is described in which there are no inter-pair orthogonality constraints: furthermore, the extent of pair localization can be predetermined by the chosen forms for the orbital expansions of the pair functions. The one- and two-particle density functions are presented in a compact manner using a matrix formalism. Illustrative calculations are performed on three two-pair four-electron systems: the π-system of butadiene, He2 and LiH. In each case, the orbital expansions of the pair functions are very simple, and the results compare favourably with those obtained from more complex and less chemically based wave functions.