First-principles density-functional theory, together with the embedded-atom method, were used to study the self-diffusion of small clusters on the (111) surfaces of 8 face-centered cubic metals. A zig-zag motion was found to be important in dimer and tetramer diffusion. The dimer diffused via a zig-zag and concerted motion. The trimer diffused via a concerted 3-atom motion. The tetramer diffused via a zig-zag motion in which only 2 atoms moved simultaneously at each step. Therefore, instead of increasing, the migration energy was decreased (or remained the same) for the tetramer; as compared with that for the trimer. This unexpected change in the generally upward trend in migration energy was suggested to be a widespread phenomenon.

Self-Diffusion of Small Clusters on FCC Metal (111) Surfaces. C.M.Chang, C.M.Wei, S.P.Chen: Physical Review Letters, 2000, 85[5], 1044-7