Multiatom interactions between hydrogen atoms near a metallic surface

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C (2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.