Relativistic many-body calculations of energies of n=3 states of Be-like ions

Energies of the 2s3s (S-1(0), S-3(1)), 2S3p (P-1(1), P-3(J)), 2s3d (D-1(2), D-3(J)), 2p3s (P-1(1), P-3(J)), 2p3p (S-1(0), S-3(1), P-1(1), D-1(2), P-3(J), D-3(J)) and 2p3d (P-1(1), D-1(2), F-1(3), P-3(J), D-3(J), F-3(J)) states for Be-like ions with nuclear charges Z in the range 4-30 and for Z = 54 are determined to second order in relativistic many-body perturbation theory starting from a frozen-core Dirac-Fock basis. Second-order Coulomb and second-order Breit-Coulomb correlation energies are evaluated. Corrections for the frequency dependence of the Breit interaction are taken into account in lowest order. Lamb-shift corrections to the energies are also included in lowest order. Comparisons are made with other theoretical calculations and with experimental data. We obtain an excellent agreement with experiment for term splitting, even for low-Z ions. Results for the 2p-3d excitation energies for Xe50+ are also in very good agreement with experiment.