Small Au clusters on a defective MgO(100) surface

The lowest energy structures of small Au_N clusters (N = 4, 6, 8, 10) adsorbed on an MgO(100) terrace presenting a double vacancy (DV) neutral defect are investigated via a Density-Functionl Basin-Hopping (DF-BH) approach. Gas-phase gold clusters in this size range exhibit planar structures, while a crossover from planar to cage structural motifs takes place between size 8 and 10 when the clusters are adsorbed on the defected surface. A fluxional behavior is found with different structural motifs very close in energy, in agreement with the co-existence of different epitaxies observed in Molecular Beam Epitaxy experiments on larger clusters. Au_6, that is energetically favored in the gas-phase with respect to neighboring sizes thanks to an electronic shell-closure effect, presents a reduced HOMO-LUMO gap, and its stability is comparable to that of other even clusters.