Adsorption of n-butane on Cu(100), Cu(111), Au(111), and Pt(111): Van der Waals density-functional study

The adsorption of n-butane on Cu(100), Cu(111), Au(111), and Pt(111) is studied as a prototypical physisorption system of organic molecule-metal interface by using a fully nonlocal Van der Waals density functional (vdW-DF) [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] and a second version of it (vdW-DF2). Adsorption energies and heights are compared with experiments and conventional local (local-density approximation) and semilocal (generalized gradient approximation) functionals. The adsorption energies and heights predicted by vdW-DF2 agree most favorably with known estimates. Lateral intermolecular interactions at a full coverage are calculated to be about 25% of total adsorption energy. A summation up to the second-nearest-neighbor pairs, i.e., six contacting neighbors, is enough to make the pairwise sum converged.