Nature of the Darwin term and ${(Z\alpha)^4 m^3/M^2}$ contribution to the Lamb shift for an arbitrary spin of the nucleus

Abstract

The contact Darwin term is demonstrated to be of the same origin as the spin-orbit interaction. The $(Z\alpha)^4 m^3/M^2$ correction to the Lamb shift, generated by the Darwin term, is found for an arbitrary nonvanishing spin of the nucleus, both half-integer and integer. There is also a contribution of the same nature to the nuclear quadrupole moment.