A detailed ring polymer study was made of the diffusion of hydrogen on Ni(100) using the embedded atom method interaction potential. Particular attention was paid to the effects of lattice motion, transition state re-crossing and multiple hops. It was shown that all these effects could be assessed within a unified theoretical framework Firstly, a study was made of the low-temperature regime where the diffusion coefficient could be calculated by the random walk model. The crossover from thermally activated diffusion to almost temperature-independent quantum diffusion was found at around 70K, in agreement with earlier quantum instanton calculations. It was shown that the re-crossings of the transition state dividing surface became significant only below the crossover temperature. The lattice motion slightly increased the diffusion coefficient above, and slightly decreased it below, the crossover temperature. It was also shown that quantizing the motion of the metal atoms had a negligible effect, even at very low temperatures. These last two observations were at variance with previous theoretical results obtained using the same interaction potential. It was argued that this was primarily due to the different lattice models employed in the various calculations. Secondly, a study was made of the high-temperature regime. The diffusion coefficients were computed using the Einstein and Green-Kubo relationships. Comparison of these results with those generated by the random walk model permitted examination of the role of correlated dynamic events in diffusion. A noticeable contribution of correlated rebound events in the Einstein and Green-Kubo calculations was found, leading to a decrease in the diffusion coefficient as compared to the random walk estimate at above 300K.

Surface Diffusion of Hydrogen on Ni(100) from Ring Polymer Molecular Dynamics. Suleimanov, Y.V.: Journal of Physical Chemistry C, 2012, 116[20], 11141-53