Retarded Boson-Fermion interaction in atomic systems

The retarded interaction between an electron and a spin-0 nucleus, that has been derived from electro-dynamical perturbation theory is discussed here. A brief account of the derivation is given. The retarded form is correct through order v ²/c ². Use of the relative coordinates leads to an effective one-electron operator that can be used through all orders of perturbation theory. A few unitary transformations give rise to the interaction that is valid in the non-relativistic limit.