Intermediate Hamiltonian Fock-space coupled-cluster method: Excitation energies of barium and radium

An intermediate Hamiltonian Fock-space coupled cluster method is introduced, based on the formalism developed by Malrieu and co-workers in the context of perturbation theory. The method is designed to make possible the use of large P spaces while avoiding convergence problems traceable to intruder states, which often beset multireference coupled cluster schemes. The essence of the method is the partitioning of P into a main P-m and an intermediate P-i serving as buffer, with concomitant definition of two types of wave and excitation operators. Application to atomic barium and radium yields converged results for a large number of states not accessible by traditional Fock-space coupled cluster. Moreover, states calculated by both methods exhibit better accuracy (by a factor of 2-5) in the intermediate Hamiltonian approach. Energies are given for low-lying states of Ra which have not been observed experimentally. (C) 2000 American Institute of Physics. [S0021-9606(00)30246-X].