  Collective diffusion coefficient in a two-dimensional lattice gas on a
nonhomogeneous substrate is investigated using variational approach. Particles
reside at adsorption sites with different well depths potentials and jump
randomly between them. The site blocking is the only particle--particle
interaction mechanism. It is shown that the value of the diffusion coefficient
in one lattice direction depends nontrivially on the rate and the character of
the particle jumps in all directions. When the jump rate in the direction
perpendicular to that along which the diffusion is observed increases, the
collective diffusion coefficient approaches values predicted within the mean
field approximation. Results of the Monte Carlo simulations for selected
systems are very well reproduced by our analytical results.
