A study of quasidegeneracy effects in the be-isoelectronic series

Abstract

Coupled-pair (CPMET), doubly-excited configuration interaction (D-CI) and linear coupled-pair (LCPMET) calculations on several members (Li−, Be, B+, C2+, Ne6+, Ar14+, Zn26+) of the Be-isoelectronic series are reported and compared with other methods such as perturbation theory. Special attention is given to the reliability of coupled-pair methods for both positive and negative ions in the presence of quasidegeneracy. The impact of disconnected quadruply excited configurations is significant for Li− and decreases considerably for the positive ions as the nuclear charge increases. The present single-reference state results indicate that coupled-pair methods are considerably more accurate than perturbation theory in the quasidegenerate systems considered. It is also emphasized that the description of quasidegeneracy effects should take into account both the energies of the configurations involved and the relative magnitudes of these configurations in the wave function.