Cluster expansion of the wavefunction. Symmetry-adapted-cluster expansion, its variational determination, and extension of open-shell orbital theory

作者：Hiroshi Nakatsuji, Kimihiko Hirao · 发表于：The Journal of Chemical Physics · 年份：1978 · DOI：10.1063/1.436028 · 被引用次数：657 · 研究领域：Advanced Chemical Physics Studies、Chemical Thermodynamics and Molecular Structure、Porphyrin and Phthalocyanine Chemistry

The symmetry-adapted-cluster (SAC) expansion of an exact wavefunction is given. It is constructed from the generators of the symmetry-adapted excited configurations having the symmetry under consideration, and includes their higher-order effect and self-consistency effect. It is different from the conventional cluster expansions in several important points, and is suitable for applications to open-shell systems as well as closed-shell systems. The variational equation for the SAC wavefunction has a form similar to the generalized Brillouin theorem in accordance with the inclusion of the higher-order effect and the self-consistency effect. We have expressed some existing open-shell orbital theories equivalently in the conventional cluster expansion formulas, and on this basis, we have given the pseudo-orbital theory which is an extension of open-shell orbital theory in the SAC expansion formula.