An Ornstein Uhtenbeck process in a periodic potential in R^d is considered. It has been shown previously that this process satisfies a central limit theorem in the sense that, by rescaling space and time in a suitable way, the distribution of the process converges to that of a Wiener process with nonsingular diffusion matrix. Here a rigorous proof is given of a version of Einstein's formula for this model, relating the diffusion constant to the "mobility" of the system.