A comparison of the Kubo-Green and fluctuation methods for determining the surface diffusion coefficients of adsorbates was carried out by performing Monte Carlo simulations of a square lattice with various adsorbate-adsorbate nearest-neighbor and next-nearest neighbor interactions. It was found that, even in the ordered regions of the phase diagrams, both methods agreed when the system was fully equilibrated. They almost agreed, even if the system was not fully equilibrated, provided that the so-called local mean-square fluctuation was used in the Kubo-Green method for the latter situation. Both the mean-square fluctuations and the diffusivity were extremely sensitive, even to small deviations from equilibrium, at a coverage close to 0.5 and at temperatures below the critical temperature. It was also found that the tracer and jump diffusion coefficients were almost identical under all conditions. The present work was made possible by the use of a particularly powerful computer which permitted both diffusion and equilibration simulations to be carried out simultaneously in the canonical ensemble. A discrepancy between the 2 methods, which had been noted at low temperatures in previous work (where mean-square fluctuations were found in the grand canonical ensemble and diffusion was found in the canonical ensemble) was thereby resolved.

C.Uebing, R.Gomer: Journal of Chemical Physics, 1994, 100[10], 7759-66