Effect of the complete Breit interaction on two-electron ion energy levels

Abstract
The effect of the fully relativistic Breit term upon atomic energy levels is investigated. The gauge dependence of the retarded electron-electron interaction in a self-consistent-field scheme is studied both theoretically and numerically. The complete Breit operator mean values are computed in Lorentz gauge and compared with those of the semirelativistic retarded interaction in covariant and radiation gauges for high-Z heliumlike ion singly excited states. The off-shell energy case is considered in detail even for a perturbative approach.