Noble gases on metal surfaces: Insights on adsorption site preference

We use a nonlocal van der Waals density functional (vdW-DF) approach to reexamine the problem of why noble gases are experimentally observed to adsorb on low-coordination atop sites rather than on high-coordination hollow sites for several different metal surfaces. Previous calculations using density functional theory (DFT) within the local density approximation (LDA) ascribed the site preference to reduced Pauli repulsion at atop sites, largely due to reduced exchange repulsion within LDA-DFT. In contrast, our vdW-DF calculations show that site preference is not due to differences in the exchange repulsion at all, but rather the result of a delicate balance between the electrostatic and kinetic energies; surprisingly, exchange-correlation energies play a negligible role in determining site preference. In contrast to previous calculations, we find that experimental results cannot be explained in terms of binding energy differences between atop and hollow sites. Instead, we show that the hollow sites are transition states rather than minima on the two-dimensional potential energy surface, and therefore not likely to be observed in experiments. This phenomenon is quite general, holding for close-packed and non-close-packed metal surfaces. We show that inclusion of nonlocal vdW interactions is crucial for obtaining results in quantitative agreement with experiments for adsorption energies, equilibrium distances, and vibrational energies.