Analysis of van der Waals density functional components: Binding and corrugation of benzene and C-60 on boron nitride and graphene

The adsorption of benzene and C60 on graphene and boron nitride is studied using density functional theory with the van der Waals density functional (vdW-DF). By comparing these systems we can systematically investigate their adsorption nature and differences between the two functional versions vdW-DF1 and vdW-DF2. The bigger size of the C60 molecule makes it bind stronger to the surface than benzene, yet the interfaces between the molecules and the sheets are similar in nature. The binding separation is more sensitive to the exchange variant used in vdW-DF than to the correlation version. This result is related to the exchange and correlation components of the potential energy curve. We show that a moderate dipole forms for C60 on graphene, unlike for the other adsorption systems. We find that the corrugation (at the atomic scale) is very sensitive to the variant or version of vdW-DF used, in particular, the exchange. Further, we show that this sensitivity arises indirectly through the shift in binding separation caused by changing the vdW-DF variant. Based on our results, we suggest a concerted theory-experiment approach to assess the exchange and correlation contributions to physisorption. Using DFT calculations, the corrugation can be linked to the optimal separation, allowing us to extract the exchange-correlation part of the adsorption energy. Molecules with the same interfaces to the surface, but different geometries, can in turn cast light on the role of van der Waals forces.