Exchange-correlation and QED effects from a density functional based level shift approach

In the present contribution an alternative systematic approach to this functional is put forward, which is based on a density functional version of Sucher's level shift formula. As in the case of the coupling constant integration form, an exact relation for the xc-functional can be established. This relation provides the basis for a perturbation expansion to second order, including all inherent radiative corrections. A detailed comparison with the coupling constant integration result verifies the equivalence of both approaches. Finally, the first order level shift is analyzed in order to identify the Lamb shift within the Kohn-Sham scheme.