State-specific multireference coupled-cluster theory of molecular electronic excited states
A state-specific coupled cluster (CC) theory with the CAS (complete active space) reference (CASCC) and based on an approach that employs single reference determinant (so-called “formal reference” determinant) in the expression for the wave function has been developed to study electronic excited states with different spatial and spin symmetries. The formal reference determinant is used to generate the reference wave function in the form of a linear combination of the CAS determinants contracted to configurations with the spin and spatial symmetries of the target state. Such properly symmetrized multideterminantal reference provides the zero-order description of the state under consideration. To that reference an exponential CC operator is applied to describe the dynamic electron correlation effects in the CASCC wave function of the considered state. All necessary equations for the CASCC energy and configurational amplitudes are generated using an automated computer-based method which constructs all necessary coupled cluster diagrams for any arbitrary level of the electronic excitation.