Strain - induced interaction energies between hydrogen atoms in palladium

We have made quantitative calculations of the elastic interactions between interstitial hydrogen atoms in Pd metal. These calculations use the Harwell HADES code, and hence go beyond the usual harmonic models. Results have been obtained for several potentials and, where appropriate, agree well with those of previous workers. W e find (i) that the absolute values are sensitive to assumptions for the potentials, suggesting caution in the prediction of thermodynamic properties, and (ii) that there are significant few-body terms not included in the usual approaches. These extra terms will affect the equilibrium structure. for example by removing the symmetry between fractional occupancies 0 and ( I 0). and may lead to the initial nucleation of metastable structures during hydrogenation. The present results suggest that corner-sharing tetrahedra are favoured.