Density functional study of M₄ clusters (M=Cu, Ag, Ni, Pd) deposited on the regular MgO(001) surface

The interaction of square-planar M₄ clusters (M=Ag, Cu, Ni, Pd) with the ideal MgO(001) surface has been investigated using a gradient-corrected density functional (DF) method and substrate model clusters for adsorption sites in the vicinity of O^2- centers. Polarization of the adsorbed metal clusters due to the surface electric field provides an important contribution to metal-oxide bonds. No significant elongation of the M-M bond relative to free square-planar M₄ species is calculated in adsorbed clusters, with Pd₄ exhibiting the largest expansion of 0.1 Å. At variance with Ni and Cu, for Pd and Ag only a moderate energy expense is required to sufficiently lengthen the M-M bonds such that the atoms are put in their most favorable adsorption positions on MgO close to surface O^2- anions.