Abstract
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 HH 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.
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