  Diffusion of small two dimensional Cu islands (containing up to 10 atoms) on
Cu(111) has been studied using the newly developed self-learning Kinetic Monte
Carlo SLKMC method. It is based on a database of diffusion processes and their
energetics accumulated automatically during the implementation of the SLKMC
code. Results obtained from simulations in which atoms hop from one fcc hollow
site to another are compared with those obtained from a parallel set of
simulations in which the database is supplemented by processes revealed in
complementary molecular dynamics simulations at 500 K. They include processes
involving the hcp (stacking-fault) sites, which facilitate concerted motion of
the islands. A significant difference in the scaling of the effective diffusion
barriers with island size is observed between the two cases. In particular, the
presence of concerted island motion leads to an almost linear increase in the
effective diffusion barrier with size, while its absence accounts for strong
size-dependent oscillations and anomalous behavior for trimers and heptamers
while the diffusion coefficients (calculated at 300 K, 500 K, and 700 K) do not
display any characteristic scaling with size. We also identify and discuss in
detail the key microscopic processes responsible for the diffusion and examine
the frequencies of their occurrence, as a function of island size and substrate
temperature.
