A Coupled Cluster Approach to the Electron Correlation Problem Using a Correlated Reference State

Abstract

The Coupled Cluster (CC) formalism,^1,2 starting from a single reference function, has been eminently successful for treating electron correlation for closed shell systems.³ Several generalizations to encompass multireference functions, leading to CC theories for open-shells, have been suggested in recent years⁴ in the framework of effective hamiltonians, which have offered interesting possiblities of computing directly energy differences of spectroscopic interest. Although originally designed for complete model spaces,⁵ it has been found necessary to develop the open-shell CC theory for incomplete model spaces^4,6 since otherwise there is often the danger of the intruder problem. It appears that the intruders can to a large extent be avoided for spectroscopic energy differences if one works with incomplete model spaces, but they remain a real menace for computing potential energy surfaces, since there are different intruders for the different regions of the surface.