Correlation problems in atomic and molecular systems. VI. Coupled-cluster approach to open-shell systems

Publisher Website
Google Scholar
Abstract
A new approach to the direct determination of excitation energies and various attachment or detachment energies, such as electron affinities and ionization potentials, is presented. It is based on the coupled-cluster ansatz for the trial wave function, generalized to the open-shell excited or ionized states in a way which enables a variation principle to be used, assuming that the closed-shell ground-state coupled-cluster wave function is known. The conditions for the applicability of this approach are discussed, and the basic formalism is derived. A technique enabling a systematic elimination of disconnected clusters in a general open-shell case is presented and used. An example pertaining to ionization processes is worked out in some detail. Finally, the relationship of the presented approach to configuration-interaction and perturbative Green's-function approaches is briefly outlined.