Fock-space coupled-cluster method in the intermediate Hamiltonian formulation: Model with singles and doubles

The intermediate Hamiltonian technique is applied to reformulate the Fock-space coupled-cluster (FSCC) method with singles and doubles. A version of FSCC designated to direct calculation of excitation energies is considered. The presented scheme offers several advantages over those used so far like simplification of the equations, smooth convergence, and the possibility of one-root calculation. Results of some illustrative numerical examples are also shown where comparison with other methods as well as with the full configuration interaction results is available.