Intermediate Hamiltonian formulation of the valence-universal coupled-cluster method for atoms

A formulation of the valence-universal coupled-cluster (VU-CC) method for atoms is presented. The method is based on Lindgren's normal-ordered exponential expansion, in which the cluster operator is restricted to its one- and two-body components represented in terms of radial amplitudes defined by the configurational excitations. The approach employs an intermediate Hamiltonian technique which replaces the effective Hamiltonian formalism used in the standard VU-CC formulation. The modified version introduces many simplifications in comparison with the standard one and, in particular, offers an efficient way of solving coupled-cluster equations. The advantages of the approach are illustrated in a calculation for the Be atom, representing one of the basic yet very challenging applications for VU-CC methods.