  The diffusion of two dimensional adatom islands (up to 100 atoms) on Cu(111)
has been studied, using the self-learning Kinetic Monte Carlo (SLKMC) method
[1]. A variety of multiple- and single-atom processes are revealed in the
simulations, and the size dependence of the diffusion coefficients and
effective diffusion barriers are calculated for each. From the tabulated
frequencies of events found in the simulation, we show a crossover from
diffusion due to the collective motion of the island to a regime in which the
island diffuses through periphery-dominated mass transport. This crossover
occurs for island sizes between 13 and 19 atoms. For islands containing 19 to
100 atoms the scaling exponent is 1.5, which is in good agreement with previous
work. The diffusion of islands containing 2 to 13 atoms can be explained
primarily on the basis of a linear increase of the barrier for the collective
motion with the size of the island.
