On v²/c² expansion of the Dirac equation with external potentials

The v^2/c^2 expansion of the Dirac equation with external potentials is reexamined. A complete, gauge invariant form of the expansion to order (1/c)^2 is established which contains two additional terms, as compared to various versions existing in the literature. It is shown that the additional terms describe relativistic decrease of the electron spin magnetic moment with increasing electron energy.