We examine the mobility of a hydrogen atom adsorbed on a Ni(100) in a temperature range (200ndash;400 K) where the motion consists of jumps between lattice sites. We view these jumps as isomerization reactions and calculate their rate constants by using the fluxndash;flux correlation function theory. We examine in detail the effect of lattice fluctuations and lattice distortion on the jumping rates and test the accuracy of several short time approximations which provide an extension of the transition state theory to quantum systems. We find that the magnitude of the diffusion coefficient is affected by multiple jumps and that recrossing effects are significant. By comparing the present quantum results to those obtained previously by classical simulations, we find that in this temperature range the quantum effects are small (i.e., at most a factor of 6) and originate mostly from the differences in the magnitudes of the thermodynamic quantities appearing in the rate coefficient expression. Numerical experiments show that it is possible to calculate the transition state rate coefficient in quantum systems by using a free particle approximation to calculate the short time evolution of the fluxndash;flux correlation function. This approximation provides significant computer time savings and will permit calculations for quantum systems with a very large number of degrees of freedom.
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