Compact surface-cluster diffusion by concerted rotation and translation

First-principles calculations show that a pseudomorphic Ir monolayer on Lr(111) has an unexpected metastable on-top configuration only slightly above the stable fee configuration. The energy required for any translation of the monolayer parallel to the surface is small. This :opens the possibility that whole clusters can move by rotation and/or translation through configurations: with many atoms in on-top sites. One such diffusion mechanism, the ''cartwheel shuffle'' could explain otherwise mysterious long jumps and high prefactors seen in the diffusion of 19-atom Ir clusters on Ir(111).