Theoretical calculations were made of adsorption, diffusion and island formation of water ad-molecules on the basal-plane surface of an ice Ih crystal. At low coverages, it was found that an ad-molecule preferred to locate at non-crystallographic sites (those that did not fit into the ice lattice) on the surface. Since ice Ih was proton-disordered, no two sites were exactly the same and there was a wide range of binding energies. For some local environments the binding energy was of the order of, or larger than, the cohesive energy. The proton disorder also resulted in a range of activation energies for diffusion. After mapping out a large number of diffusion barriers using the nudged elastic band method, a kinetic Monte Carlo calculation was made of diffusion at 140K. For short times, the mean square displacement exhibited the anomalous scaling with time that was common for diffusion on random lattices. At longer times, the scaling was normal and a diffusion coefficient could be obtained. The energetics and dynamics of the formation of small islands on the ice surface were also studied. It was found that islands, up to and including pentamers, were non-crystallographic but hexamers were crystallographic. While the formation of a crystallographic hexamer from a non-crystallographic pentamer and an ad-molecule involved complicated concerted motion of all of the island molecules plus a large relaxation of the substrate, the activation energy for the process was estimated to be smaller than the ad-molecule diffusion barrier.

Diffusion and Island Formation on the Ice Ih Basal Plane Surface. E.R.Batista, H.Jónsson: Computational Materials Science, 2001, 20[3-4], 325-36