Asymptotic diffusion coefficient of particles in a random medium

Avramov, Milchev, and Argyrakis [Phys. Rev. E 47, 2303 (1993)] have investigated the mean-square displacements of particles that perform random walks in two- and three-dimensional lattices with random barriers with uniform distributions of activation energies. They discussed the crossover between anomalous and normal diffusion, but they did not analyze the behavior of the mean-square displacements at long times where normal diffusion occurs. We point out that the asymptotic diffusion coefficients are well described by the effective-medium theory in the range of parameters investigated; they are in disagreement with the predictions of critical-path arguments in dimension d=3.