Variational localized-site cluster expansions. I. General theory

Abstract

Cluster expansions in terms of general sites (which may be spin orbitals, spin‐free orbitals, shells, component atoms in a molecule, etc.) are developed for electronic wavefunctions. Emphasis is placed upon the potential simplification of such expansions in the case that the sites are localized. Formulas for both diagonal and off‐diagonal matrix elements for such cluster expanded wavefunctions are given, and approximations to these general formulas are discussed.