The diffusivity of H2 on a clean (111) surface, at temperatures ranging from 50 to 150K, was calculated by using molecular dynamics simulations. The H2 molecule was treated not only as a rigid-rotor, but also as an entity with vibrational motion. Phonon-molecule coupling was also considered. The ensemble-averaged square displacement, as well as the velocity auto-correlation function, were used over a range of energies in order to deduce the diffusion coefficients. The results of the simulation were compared with the results of calculations which were based upon simple transition state theory. It was shown that the latter results underestimated the diffusion coefficient, as compared with the simulations.

L.Wang, Q.Ge, G.D.Billing: Surface Science, 1994, 301[1-3], 353-63