APPLICATION OF THE COMPLETE-MODEL-SPACE MR-CCSD THEORY TO THE 2S(2) S-1-STATES AND 2P(2) S-1-STATES OF BE

A complete-model-space (CMS) valence-universal coupled-cluster (VU-CC) method is applied to the problem of calculating the energies and wavefunctions for the 2S2 1S and 2p2 1S quasidegenerate states of Be. The difficulties encountered with this problem have supported the viewpoint that CMS formulations of CC theories are inapplicable in the presence of intruder states. The present approach is based on Lindgren's normal ordered Ansatz for the wave operator. The cluster operator is approximated by its one- and two-electron components represented in terms of configurational excitations, which results in a drastic reduction of cluster amplitudes compared with the spin-orbital representation. The results obtained for a 9s7p5d STO basis set are in good agreement with their accurate counterparts. The multi-reference CC formulation improved the ground state correlation energy by 1.3% in comparison with the single-reference approach.