Quantum and classical calculations of equilibrium rate constants for surface diffusion were carried out using a model of CO adsorbed on Ni(111). Quantum transition-state theory rates were calculated using a Monte Carlo path integral method. Arrhenius plots of both the quantum and classical rate constants were linear from 100 to 300K. Over this range, the quantum activation energy was slightly lower than the classical value due to a difference between zero-point energies in the reactant and transition states. At below 100K, quantum rates were significantly higher than expected from the high-temperature Arrhenius plot. This was the result of quantum tunneling effects. The density matrix elements used to derive the quantum transition-state theory rate were also used to illuminate the differences between classical and quantum mechanics for this model.

Quantum Effects in Elementary Surface Reactions: CO Diffusion on Ni(111). A.Calhoun, D.Doren: Journal of Physical Chemistry, 1993, 97[10], 2251-6