Generalization of the concept of Brueckner orbitals for multi-reference-state methods

Abstract

The concept of maximum overlap or Brueckner orbitals is generalized to the case of multi-reference-state formulations of many-electron theory by considering the maximum proximity orbitals (MPOs) that yield the maximum proximity of the target ( M ) and model ( M 0) spaces. To understand better the impact of the distance of the M 0 and M spaces on MR-type approaches, model studies have been undertaken for the minimum basis set H 4 system. Proximity indices for several standard quantum-chemical orbital sets are compared with those for the MPOs. The MPOs are also applied in calculations based on MR-CC approaches of valence-universal and state-universal types.