Benzene on Cu(111): I. Application of van der Waals-Density Functional Formalism to Determine Binding Sites and Energy Contour Map

With a recently developed van der Waals density functional (vdW-DF)\footnote{M. Dion et al., Phys. Rev. Lett. 92 (2004) 246401} we study the adsorption of benzene on Cu(111).\footnote{KB, TLE, and PH, Phys. Rev. B 80 (2009) 155431} The vdW-DF inclusion of nonlocal correlations changes the relative stability of 8 high-symmetry binding-position options and increases the adsorption energy by over an order of magnitude, achieving good agreement with experiment. The metallic surface state survives benzene adsorption. From a contour plot of the potential energy, we find that benzene can move almost freely along a honeycomb web of ``corridors" linking fcc and hcp sites via bridge sites, consistent with the low diffusion barrier in experiment.