The coupled pair functional (CPF). A size consistent modification of the CI(SD) based on an energy functional

Abstract

A modification of the CI(SD) energy functional is proposed which leads to size consistency through the use of partial normalization denominators. The method is derived from simple principles: correct description of separated two-electron systems and certain invariance requirements. This approach is connected to CEPA-1. The theoretical framework allows for a simple rationalization of connections between CI(SD), CEPA-1, and the linear version of CP–MET. As demonstrative applications we report comparisons with full CI calculations for BH, NH3, H2O, HF, and Re, De for F2, N2, O2, Cl2, NO, and CO obtained for very large basis sets.