Multireference self-consistent size-consistent singles and doubles configuration interaction for ground and excited states

One proposes a state-specific self-consistent dressing of the configuration interaction (CI) matrix built on a multireference space and all the singly and doubly substituted determinants. The dressing insures size consistency [and separability when localized molecular orbitals (MOs) are used]. In the here-proposed solution, which generalizes a previous single reference method [(SC)2CI, J. Chem. Phys. 99, 1240 (1993)] valid only for the research of the ground state, all the reference determinants play an equal role and the method is applicable to excited states. The implementation will be simpler if the reference space is a complete active space, but this restriction is not compulsory.