Pd and Ag dimers and tetramers adsorbed at the MgO(001) surface: a densityfunctional study

We have studied computationally the adsorption properties of small Ag and P d clusters deposited on the MgO(001) surface. The calculations were carried out employing a gradient-corrected density functional approach; the oxide surface was represented by model clusters embedded in a large array of poin t charges. Supported Ag and Pd dimers and tetramers were investigated in or der to identify the preferred adsorption sites and the modifications induce d in the cluster by the interaction with the substrate. All metal clusters adsorb in proximity of oxygen centers. An adsorption mode with the molecula r axis parallel to the surface is the most stable one for Pd-2 while Ag-2 p refers an upright adsorption mode. Various isomer structures of the support ed metal tetramers have been considered. In general, the most stable gas ph ase structure is also the preferred one upon adsorption. This suggests that the metal-metal bonding prevails over the metal-MgO interaction.