Surface self-diffusion constants at low temperature: Monte Carlo transition state theory with importance sampling

We present an importance-sampling method which, when combined with a Monte Carlo procedure for evaluating transition state theory rates, allows computation of classically exact, transition state theory surface diffusion constants at arbitrarily low temperature. In the importance-sampling method, a weighting factor is applied to the transition state region, and Metropolis steps are chosen from a special distribution which facilitates transfer between the two important regions of configuration space: the binding site minimum and the saddle point between two binding sites. We apply the method to the diffusion of Rh on Rh(111) and Rh on Rh(100), in the temperature range of existing field ion microscope experiments.