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 s tates, which often beset multireference coupled cluster schemes. The essenc e of the method is the partitioning of P into a main P-m and an intermediat e P-i serving as buffer, with concomitant definition of two types of wave a nd excitation operators. Application to atomic barium and radium yields con verged results for a large number of states not accessible by traditional F ock-space coupled cluster. Moreover, states calculated by both methods exhi bit better accuracy (by a factor of 2-5) in the intermediate Hamiltonian ap proach. Energies are given for low-lying states of Ra which have not been o bserved experimentally. (C) 2000 American Institute of Physics. [S0021-9606 (00)30246-X].