We introduce a novel empirical model for the adhesion of Pd clusters on the MgO{100} substrate. The new model corrects the known bias of previous models toward structures with large interfaces with the substrate due to the failure to account for the so-called "metal-on-top" effect, i.e., the enhancement of the adhesion due to the presence of other metal atoms on top of those which are directly in contact with the substrate. The new model is parametrised using density-functional theory calculations on MgO-supported Pd clusters with sizes up to 80 atoms. The proposed potential is continuous with respect to spatial coordinates and can therefore be used directly in molecular dynamics simulations.