1Helsinki Institute of Physics, University of Helsinki,
2Laboratory of Physics, Helsinki University of Technology, Espoo, Finland
3Department of Physics, University of Jyväskylä, Jyväskylä, Finland
4Department of Physics, Brown University, Providence RI 02912
We present Monte Carlo simulations for the size and temperature dependence of the diffusion coefficient of adatom islands on the Cu(100) surface. We use a lattice-gas model where the interaction parameters have been determined by effective medium theory calculations . We show that the scaling exponent for the size dependence is not a constant but a decreasing function of the island size and approaches unity for very large islands. This is due to a crossover from periphery dominated mass transport to a regime where vacancies diffuse inside the island. The effective scaling exponents are in good agreement with theory  and experiments .