  We develop a practical method of computing the stationary drift velocity V
and the diffusion coefficient D of a particle (or a few particles) in a
periodic system with arbitrary transition rates. We solve this problem both in
a physically relevant continuous-time approach as well as for models with
discrete-time kinetics, which are often used in computer simulations. We show
that both approaches yield the same value of the drift, but the difference
between the diffusion coefficients obtained in each of them equals V*V/2.
Generalization to spaces of arbitrary dimension and several applications of the
method are also presented.
