A VALENCE-UNIVERSAL COUPLED-CLUSTER SINGLE-EXCITATIONS AND DOUBLE-EXCITATIONS METHOD FOR ATOMS .2. APPLICATION TO BE

The recently formulated atomically oriented form of the valence-universal coupled-cluster theory accounting for one- and two-electron excitations (VU-CCSD/R) is applied to several states of Be. The complete model space (CMS) is employed. Various approximate forms of this method including the lowest-order perturbation theory are also considered. The standard single-reference (SR) formulation of the CCSD method, treated as a special case of the VU-CCSD/R approach, is applied to the ground state. The calculations for the 9s9p7d7f5g STO basis set are the most accurate ones performed so far for any SR- and VU-CCSD method formulated in terms of analytical orbitals. Proceeding from the SR- to the VU-formulation increased the magnitude of the ground-state correlation energy by 1.1%. Special attention is paid to the well known intruder state problem for the pair of 2s2 1S and 2p2 1S states. It is demonstrated that, despite earlier findings, the cms based VU-CCSD methods can be safely used in the presence of intruder states. The impact of the electronic structure of the series of systems considered in VU-CCSD calculations on the accuracy of results is also addressed.