  The adsorption of benzene and C60 on graphene and boron nitride (BN) is
studied using density functional theory with the non-local correlation
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 interface 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 (PEC). We show that a moderate dipole forms for C60 on graphene, unlike
for the other adsorption systems. We find that the corrugation is very
sensitive to the variant or version of vdW-DF used, in particular the exchange.
Further, we show that this sensitivity arise indirectly through the shift in
binding separation caused by changing 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 same
interfaces to the surface, but different geometries, can in turn cast light on
the role of van der Waals forces.
