Compact Variational Wave Functions Incorporating Limited Triple and Quadruple Substitutions

摘要: We have investigated a number of configuration interaction (CI) wave functions which incorporate the dominant effects triple and quadruple substitutions. If we employ natural orbitals from CI singles doubles (CISD) procedure, then may be conveniently partitioned into subspaces varying importance, electron configurations classified according to how they occupy each subspaces. model space containing all single double substitutions also those classes triply quadruply substituted deemed most important on basis orbital populations. This priori selection scheme recovers large fraction energy including triples quadruples (CISDTQ) at drastically reduced computational expense, it represents more “black-box” alternative traditional multireference approaches. Implications for nonvariational approaches are discussed.