Intermediate Hamiltonian Fock-space coupled-cluster method

An intermediate Hamiltonian Fock-space coupled-cluster method is presented, following the formalism developed by Malrieu in the context of perturbation theory. The method is expected to be applicable to many states not accessible by traditional FSCC. A pilot application to a large number of excited states of Sc⁺ shows good convergence and agreement with experiment to a few hundredths of an eV, whereas the usual FSCC iterations diverge for all but the few lowest states.