Structure and Dynamics of Small Metallic Clusters on an Insulating Metal-Oxide Surface: Copper on MgO(100)

We report density functional calculations of the structural and dynamical properties of small copper clusters ( Cuₙ, n0ex0ex=0ex0ex2--5) adsorbed on MgO(100). As Cu-Cu intracluster interactions are stronger than cluster-surface interactions, the most stable geometries are found to be similar to those in the gas phase, with the cluster in an upright position with respect to the surface. Further ab initio molecular dynamics simulations indicate that the adsorbed clusters diffuse by ``rolling'' and ``twisting'' motions, with barriers that may be smaller than that for the hopping diffusion of a single adatom.