Crystal-face dependence of physisorption potentials

We have measured a crystal-face dependence of the physisorption energies for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ interacting with the Cu(111), Cu(100), and Cu(110) surfaces. The trend of potential-well depths, ${\mathit{D}}_{111}$<${\mathit{D}}_{100}$<${\mathit{D}}_{110}$, is the opposite of that predicted by existing theory. We argue that a larger binding energy for an open crystal face is a consequence of the face-dependent electron-density profiles and their dynamical response, in particular, the influence on the attractive van der Waals interaction.