North-Holland INDIRECT INTERACTIONS OF H/Ni(lll) AND H/Pd(lOO) USING EMBEDDED ATOM METHOD

Using the embedded atom method (EAM), we computed lateral interactions between H atoms on Ni(ll1) and on Pd(100). For H/Ni(lll) the binding-energy difference between the two 3-fold sites is very small. As expected due to the very small separation, at nearest neighbor sites there is an extreme repulsion. The only other non-negligible interactions are repulsions of comparable size (a few mev) between H pairs at 2nd and 3rd neighbor separations. Energies for ordered patterns are consistent with isolated pairs: multi-site effects are not significant. Zero-point motion weakens the repulsions for isolated pairs but not for ordered overlayers. Local relaxations are minor. There are no anomalous attractions, but interactions are much smaller than expected from experiment, and (2 x 1) ordering is predicted instead of the observed graphitic (2 x 2). Recent extensions of EAM suggest ways to compute more reliable magnitudes. For H/Pd(lOO) adsorption is predicted in center sites, but slightly below the surface plane rather than above. The interactions have realistic signs and orders of magnitude, but (2 x 1) ordering is predicted rather than the observed c(2 X 2). 1