The Effect of Nuclear Motion in the Dirac Equation

Corrections to the Dirac equation, relativistic to the order $\frac{v^2}{c^2}$, due to the motion of the nucleus, for the case of an electron in the field of a heavy particle of mass $M$, are derived with the aid of the Breit two-body relativistic interaction, to the order $\frac{m}{M}$, $m$ being the mass of the electron. A calculation of the value of the correction for a $1s$ electron gives a result in agreement with that obtained from the Schrödinger treatment to the order $\frac{m}{M}$.