The spin optimized separated pair method

Abstract

The separated pair method, for systems containing even and odd numbers of electrons, is generalized by allowing for contributions from all available spin coupled states of the same total spin quantum numbers S and M. The total wavefunction is expressed as a linear combination of products of a given spin eigenfunction, which is constructed out of singlet and triplet coupled pairs, and a suitable spatial function built out of symmetric (for singlet) and antisymmetric (for triplet) spatial pair functions. The electron and spin density functions are then presented in a ‘ sum of pair ’ densities form, so enabling the effects of spin optimization on the chemical bonding to be interpreted in a simple and obvious way. Finally, a discussion is given of the practical application of the spin optimized separated pair method.