Multireference Fock-space coupled-cluster and equation-of-motion coupled-cluster theories: the detailed interconnections.

The multireference Fock-space coupled-cluster (FS-CC) equations constructed via the effective Hamiltonian approach are reduced to those obtained through a partitioning technique for the matrix diagonalization. This allows finding a better route to solve the FS-CC equations for the (1,1) sector by the direct diagonalization of the properly constructed matrix, which is identical to that obtained with the intermediate Hamiltonian approach. The detailed connections with the equation-of-motion CC (EOM-CC) approach also provides alternative routes for studying the higher-sectors of Fock space, and for the treatment of properties other than the energy. The approach studied in this work is formulated at the CC singles, doubles (CCSD) and CC singles, doubles, triples (CCSDT) level. The performance of the method is illustrated by numerical examples for the C(2), C(2)H(4), and H(2)CO molecules in comparison with the EOM-CC method and available experimental data.