Energies and Z-expansion coefficients for the D states in the helium sequence

Abstract

Non-relativistic variational energies for the 1s3d(^1,3D) states of the helium isoelectronic sequence are calculated with a 50-term correlated basis set for Z=2-10, where Z is the nuclear charge, and the corresponding Z-expansion coefficients found through sixth order by fitting the energies to a series in Z^-1. A variational-perturbation calculation of these coefficients through eleventh order (with the same basis set) is presented for comparison, and agreement is satisfactory. The 3d energies obtained by summing the Z-expansion perturbation series are found to yield excellent approximations to the variational values. The calculations are extended to the 1s4d(^1,3D) states to furnish variational energies for neutral helium and perturbation energies for the higher sequence members. All of these results are the most accurate yet reported.