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

I. B. Khriplovich, A. I. Milstein, R. A. Sen'kov

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…