Virtual Delbruck scattering and the Lamb shift in light hydrogenlike atoms

We return to the problem of evaluation of the light-by-light contribution to the energy levels of the hydrogen atom. We find an additional contribution directly related to the Delbruck scattering amplitude. The new correction is larger than the previously included light-by-light terms at order alpha(2)(Z alpha)(6)ln(Z alpha) m(e). We consider the effective potential in position space using an effective field theory approach and evaluate light-by-light corrections to the energy levels of states with nonzero orbital momentum as well as to the weighted difference of s states. We also determine the large distance asymptotic behavior of the effective potential induced by the light-by-light scattering in muonic atoms.