Unitary Multiconfigurational Coupled-Cluster Method. Theory and Applications

摘要

The use of cluster expansions to describe electron correlation in molecules has become increasingly prevalent in the field of quantum chemistry and has been the subject of recent review articles. In this paper, we propose a new variant of the coupled-cluster theory which admits multiconfigurational reference functions of the complete-active-space (CASSCF) variety, provides a compact size-extensive and approximately variational description of the electronic energy, and is capable of incorporating certain aspects of the so-called unitary group approach (UGA). Despite recent advances in the theory and application of coupled-cluster theories has been notably slower. The need for a multideterminental theory is manifest, especially for studies of potential energy hypersurfaces on which the configurations which provide the essential (or qualitative) description of the wave function change with nuclear coordinates. Reprints. (MJM)