Multireference self-consistent size-extensive state-selective configuration interaction

In this work, we propose a state-specific self-consistent ''dressing'' of the multireference configuration interaction (MRCI) space to include all single- and double-substituted determinants for the most important reference configurations. The aim of the method is to provide a size-extensive description of the dynamic electron correlation effects for states which mandate a multideterminantal reference wave function. Such states can represent electronic excited states or ground states of the molecular systems which are significantly deformed from their equilibrium structures, The proposed approach follows the concept introduced in our recently proposed quasilinear ansatz for the state-selective multireference coupled-cluster method. The purpose of the dressing procedure is to eliminate the contributions which introduce size-extensivity violating terms in the MRCI approach. (C) 1996 American Institute of Physics.