Structures of small Au clusters on MgO(001) studied by density-functional calculations

The structure of Au clusters adsorbed on MgO(001) is studied by density-functional calculations. The results of two different exchange-correlation functionals, the local density approximation (LDA) and the Perdew-Burke-Ernzerhof (PBE) functionals, are compared. The size range 11 <= N <= 24 atoms is considered. It is shown that a complex competition among a variety of structural motifs takes place. These motifs comprise two-dimensional leaflets, open cages, tetrahedral cages, and compact three-dimensional structures. Both functionals predict qualitatively the same kind of behavior with increasing cluster size, with morphology transitions from leaflets to open cages and finally to compact structures, together with a narrow range, around N = 20, in which tetrahedral cages are dominant. However, there are quantitative differences about the sizes of the crossover, which occurs at smaller sizes according to the LDA functional with respect to the PBE functional. This result is consistent with the fact that the LDA functional overestimates metallic bonding in Au, whereas the PBE functional underestimates it.