Surface Diffusion of Hydrogen on Ni(100) from Ring Polymer Molecular Dynamics

In this paper, we present a detailed ring polymer molecular dynamics (RPMD) study of the diffusion of hydrogen on Ni(100) using the well-established embedded atom method (EAM) interaction potential. We pay particular attention to the effects of lattice motion, transition state recrossing, and multiple hops. We show that all these effects can be assessed within a unified theoretical framework using RPMD. First, we study the low-temperature regime where the diffusion coefficient can be calculated by the random walk model. The crossover from thermally activated diffusion to almost temperature-independent quantum diffusion is found at around 70 K, in agreement with earlier quantum instanton calculations. We show that the recrossings of the transition state dividing surface become significant only below the crossover temperature in our RPMD calculations. The lattice motion slightly increases the diffusion coefficient above and slightly decreases it below the crossover temperature. We also show that quantizing the motion of the metal atoms has a negligible effect, even at very low temperatures. These last two observations are at variance with previous theoretical results obtained using the same interaction potential. We argue that this is primarily due to the different lattice models employed in the various calculations. Second, we studied the high-temperature regime. The diffusion coefficients are computed using the Einstein and Green–Kubo relations. Comparison of these results with those generated by the random walk model allows us to examine the role of correlated dynamical events in the diffusion. We find a noticeable contribution of correlated rebound events in our Einstein and Green–Kubo calculations, leading to a decrease in the diffusion coefficient compared to the random walk estimate at temperatures above 300 K.