General-model-space many-body perturbation theory: The (2s 3p)P1,3 states in the Be isoelectronic sequence

The energies of the (2s 3p)P1,3 states of the Be sequence ions N iv-Ne vii are calculated, with use of a general-model-space many-body perturbation theory (MBPT) to third order. The triplet states are described accurately by a one-configuration model (P) space including only 2s 3p, but the singlets are not; the singlet-triplet separation has the right sign (singlet below triplet) but is only 30% of experiment. Incorporation of the 2p 3s configuration in the model space greatly improves the results. This incomplete, two-configuration model space yields virtually the same energies as the larger, complete model space required by other quasidegenerate MBPT methods, at one-fifth the computational cost.