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

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 (M0) spaces. To understand better the impact of the distance of the M0 and M spaces on MR-type approaches, model studies have been undertaken for the minimum basis set H4 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.