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

摘要: The Coupled Cluster (CC) formalism,1,2 starting from a single reference function, has been eminently successful for treating electron correlation closed shell systems.3 Several generalizations to encompass multireference functions, leading CC theories open-shells, have suggested in recent years4 the framework of effective hamiltonians, which offered interesting possiblities computing directly energy differences spectroscopic interest. Although originally designed complete model spaces,5 it found necessary develop open-shell theory incomplete spaces4,6 since otherwise there is often danger intruder problem. It appears that intruders can large extent be avoided if one works with spaces, but they remain real menace potential surfaces, are different regions surface.