New approach to the state-specific multireference coupled-cluster formalism

Possible forms of the coupled-cluster wave function for systems which cannot be described with a single-reference approach are explored. Within the framework of the state-specific multireference coupled-cluster theory and based on the complete active space self-consistent field (CASSCF) wave functions, several proposals are made regarding the form of the reference function and the cluster structure of the CC correlation operator. An internally contracted method provides a viable approach to describing electronic excited states.