Multireference Nature of Chemistry: The Coupled-Cluster View

Various efforts made to solve quasi-degenerate problems with the coupled cluster (CC) theory are summarized and critically analyzed. Hanrath suggested a simplification for the generalized extensivity test and generalized extensivity and size-extensivity also lead to size-intensivity when one describes intensive physical properties. Nakatsuji suggested a symmetry projector applied to nonlinear terms in the SAC expansion, filtering out all wrong-symmetry components. The JeziorskiMonkhorst (JM) MRCC ansatz shows that the target electronic states have the same number of particles as all the model space determinants do. The SS-MRCC approach of Banerjee and Simons and the SS-EOMCC/pIC-MRCC methods of Nooijen and co-workers also employ a single cluster operator acting on a multideterminantal reference function. Following the classical works of Paldus and co-workers, a spin-free SR/MR CC theory operating in the space of configuration state functions (CSF) are formulated.