First-principles calculations showed that a pseudomorphic Ir monolayer on Ir(111) exhibited an unexpected metastable on-top configuration which was only slightly above the stable face-centered cubic configuration. The energy which was required for translation of the monolayer parallel to the surface was small. This suggested the possibility that entire clusters might move by rotation and/or translation, via configurations having many atoms in on-top sites. It was proposed that one such diffusion mechanism, the so-called cartwheel shuffle, could explain the otherwise mysterious long jumps and high pre-factors which were seen in the diffusion of 19-atom Ir clusters on Ir(111).

Compact Surface-Cluster Diffusion by Concerted Rotation and Translation. J.C.Hamilton, M.R.Sørensen, A.F.Voter: Physical Review B, 2000, 61[8], R5125-8