Inclusion of some higher order effects with the self-consistent electron pairs method for correlated molecular wavefunctions

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Abstract
An efficient technique is presented for the inclusion of some higher order effects in correlated wavefunctions. Diagonal quadruple substitutions (e.g., i2j2→ a2b2) are variationally added to a self‐consistent electron pairs (SCEP) wavefunction that includes single and double substitutions relative to a closed shell reference configuration. Through the use of approximate natural orbitals, these configurations include those which are important for the size‐consistency correction at the limit of separated systems. While the number of the diagonal quadruple configurations is of the order of the number of doubles, the computational effort for including the quadruples is less than for the doubles, because of the nature of the required matrix elements and the flexibility of SCEP. Variationally additive quadruple corrections to SCEP pair energies are easily found with this new procedure.