In this paper we present a systematic and rigorous method for calculating the
diffusion tensor for a Brownian particle moving in a periodic potential which
is valid in arbitrary dimensions and for all values of the dissipation. We use
this method to obtain an explicit formula for the diffusion coefficient in one
dimension which is valid in the underdamped limit, and we also obtain higher
order corrections to the Lifson-Jackson formula for the diffusion coefficient
in the overdamped limit. A numerical method for calculating the diffusion
coefficient is also developed and is shown to perform extremely well for all
values of the dissipation.Comment: 21 PAGES, 1 FIGUR