Self-diffusion of small clusters on fcc metal (111) surfaces

We use ab initio density-functional theory supplemented with the embedded-a tom method to study the self-diffusion of small clusters on the (111) surfa ce of eight fee metals. A zigzag motion is found to be important in the dim er and tetramer diffusions. The dimer diffuses by a zigzag and concerted mo tion. The trimer diffuses by a concerted three-atom motion. The tetramer di ffuses through a zigzag motion where only two atoms move simultaneously in each step. Thus, instead of increasing, the migration energy is lowered (or stays constant) for the tetramer as compared to that for the trimer. This novel break of the upwards trend in migration energy is predicted to be a g eneral phenomenon.