Surface diffusion on a pentaerythritol tetranitrate (PETN) crystal was investigated by treating the surface diffusion as an activated process in the formalism of transition state theory. In particular, surface diffusion processes on the (110) and (101) facets, as well as diffusion between these facets, were considered. The authors were able to obtain the potential energy barriers required for PETN surface diffusion. Their results show that the (110) surface is more thermally active than the (101) surface and PETN molecules mainly diffuse from the (110) to (101) facet. These results are in good agreement with experimental observations and previous simulations.