MULTI-REFERENCE COUPLED PAIR APPROXIMATION: a state-universal approach of a CEPA type variant of MRSDCI

This contribution primarily discusses coupled electron pair (CEPA) type variants of the multi-reference single and double excitation configuration interaction (MRSDCI) method. This type of theory takes advantage of simplicity and flexibility of configuration interaction method. However they incorporate more or less the knowledge from cluster expansion method. In the early stage of this note, we review configuration interaction and cluster expansion methods briefly. As the state-selective (state-specific) approach of the CEPA type variants of MRSDCI is extensively reviewed by Szalay, this report will mainly focus our attention on the state-universal approach of the CEPA type variants of MRSDCI and will be compared with the other approach from a theoretical point of view. Reliability of computational results will be discussed.