The influence of relativistic, Breit interaction, and QED effects on the 1s^22p^2 and 2s2p^3 energy levels of Be-like ( 4≤Z≤74 ) isoelectronic sequence

The excitation energies of the Be-like ions (4 ≤ Z≤ 74) doubly excited state 1 s^22p^2 and doubly core-excited state 2s2p^3 are calculated using the multi-configuration Dirac–Hartree–Fock method. The contributions of relativistic effects, Breit interaction, and QED effects to the excitation energies are analyzed and discussed in detail. A reasonable electron correlation model has been constructed using the active set method. The electron correlation and the finite size of the atomic nucleus correction have also been appropriately considered. The calculated results agree with the available theoretical and experimental data. It is found that the contribution of relativistic effects, Breit interaction, and QED effects to the excitation energy becomes significant with increasing Z . The contribution of the relativistic effects reaches about 8.5