Journal of Physics B: Atomic and Molecular Physics
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.
Blanchard, P. and Drake, Gordon W. F.. (1973). Energies and Z-expansion coefficients for the D states in the helium sequence. Journal of Physics B: Atomic and Molecular Physics, 6 (12), 2495-2503.