Theoretical calculations were made of the adsorption, diffusion and island formation of H2O ad-molecules on the basal-plane surfaces of type-Ih crystals. The preliminary calculations were based upon the TIP4P interaction potential; a pair-wise additive potential function that was based upon point charges. At low coverages, it was found that the ad-molecule preferred to place itself at sites that did not fit into the ice lattice. Because the Ih-material 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. Proton disorder also led to a range of activation energies for diffusion. After mapping out a large number of diffusion barriers by using the nudged elastic band method, a kinetic Monte Carlo calculation was made of diffusion at 140K. At first, the mean-square displacement scaled anomalously with time, as was common for diffusion on random lattices. At longer times, the scaling was normal and a diffusion coefficient could be deduced. The diffusivity was found to be slightly higher than a previously set experimental upper bound. The energetics and dynamics of formation of small islands on the surface were also studied. It was found that islands up to, and including, pentamers were non-crystallographic. The hexamer was crystallographic. The formation of a crystallographic hexamer from a non-crystallographic pentamer, and a new ad-molecule, involved a complicated concerted motion of all of the island molecules; and a large relaxation of the substrate. The activation energy for this 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