Self-consistent treatment of the frequency-independent Breit interaction in Dirac-Fock calculations of atomic structures. II. He- and Be-like ions

We systematically study the effect of the inclusion of the frequency-indepe ndent Breit interaction-treated perturbatively as well as variationally-on Dirac-Fock total energies and [r(k)] expectation values for He- and Be-like ions using an algorithm recently described by Reiher and Hinze. Fully nume rical, highly accurate solution methods are employed throughout for solving the Dirac-Fock-Coulomb-Breit (DFCB) equations. These methods allow us to i nvestigate the change of the wavefunction upon inclusion of the Breit inter action in the self-consistent field procedure of an atomic structure progra m. The dependence of expectation values on different finite-nucleus model (FNM ) potentials for the electron-nucleus attraction is also studied. It is sho wn that, in general, the choice of the FNM for the electron-nucleus potenti al hardly affects the difference between Dirac-Fock-Coulomb (DFC) and DFCB expectation values. In the case of pointlike nuclei one should treat the Br eit interaction variationally instead of perturbatively. For energy eigenvalues, we find that the difference between the Breit inter action treated self-consistently or perturbatively is negligibly small-even for highly charged ions-if an extended nucleus model is used. The data giv en may serve as a reference for more approximate treatments involving, for example, basis-set approaches with limited basis-set size. In this extensiv e study of the SCF effect on the Breit interaction, highly accurate, variat ionally obtained DFCB total energies and [r(k)] expectation values are give n for different models of the electron-nucleus interaction.