The full three-dimensional potential energy surface of H on Pd(111) has been calculated with periodic band-structure computations using the generalized gradient approximation of density-functional theory. The fcc hollow site was found to be most stable, followed by the hcp hollow site. Excellent agreement with experimental values of the adsorption energy and the vibrational frequencies was achieved. Subsurface occupation at low coverages and low temperatures is ruled out by our results, but there are no or very low barriers for hydrogen reaching the subsurface region from the molecular gas phase, thus direct absorption is feasible at high coverages. Different ordered structures of the adsorbed hydrogen were considered, and we found that two structures with $\sqrt{3}\times \sqrt{3}R30\mathrm{}°$ symmetry are most stable at low temperatures, in agreement with experiment. Results for the adsorption energies and effective hydrogen-hydrogen interactions showed that only fcc hollow sites were occupied on the ordered structures, also in agreement with experiment.

• Received 6 April 1998

DOI:https://doi.org/10.1103/PhysRevB.58.10890