Energetics and many-particle mechanisms of two-dimensional cluster diffusion on Cu(100) surfaces

We study the energetics and stability of small Cu clusters on Cu(100) surfa ces using molecular statics combined with systematic saddle-point search me thods. We find several previously overlooked concerted many-particle proces ses that play an important role in cluster energetics. In particular, for s maller clusters there is an internal atom row shear mechanism that in some cases determines the rate-limiting step for center-of-mass motion. Our resu lts suggest specific reaction paths for experimentally observed cluster dif fusion events.