Surface diffusion rates were simulated using classical molecular dynamics in a model of CO adsorbed on Ni(111). This described the energy distribution among adsorbate modes at the transition state, energy relaxation after crossing the transition state and correlations among adsorbate modes near to the transition state. The adsorbate bending (frustrated rotation) mode was strongly coupled to lateral translational motion. This molecular mode provided an important source of energy for reaching the transition state to diffusion, and an important frictional force that dissipated excess lateral translational energy. In this model, the molecular bending mode was a more important source (and sink) of lateral translational energy than the surface at short times. This was interpreted as being a consequence of directional bonding to the surface, and should be generally important in the surface diffusion of chemisorbed molecules.

Dynamics of Molecular Surface Diffusion: Energy Distributions and Rotation–Translation Coupling. Dobbs, K.D., Doren, D.J.: Journal of Chemical Physics, 1993, 99, 10041