A size extensive energy functional derived from a double configuration interaction approach:The role of N representability conditions

摘要

Starting from a configuration interaction(CI)approach including only doubly excited configurations,the corresponding energy functional is modified by introduction of a topological factor in the normalization condition of the CI coefficients in such a way that it gets inherently size extensive.Constraints imposed by N representability conditions on the modified functional are discussed and lead to a specific choice of the topological factor.The basic variable in the modified energy functional is the second-order reduced density matrix parametrized in terms of CI-like coefficients.Test calculations for a variety of small molecules show that the numerical results obtained with the new functional are in very good agreement with those obtained from coupled cluster singles doubles calculations.