Analysis of van der Waals density functional components: Binding and corrugation of benzene and C60on boron nitride and graphene

Abstract

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.

Interpretation of van der Waals density functionals

“…7 of Ref. [37] and shows the atomic configuration and contours of the electron-density distribution of the molecular building blocks for a benzene molecule and a graphene sheet at the vdW-DF binding separation. We note that a natural delineation surface of minimum electron distribution (here illustrated by a dashed curve) runs through inter-fragment positions with a saddle-point or trough-like behavior in the electron concentration.…”

Spontaneous strains and gap in graphene on boron nitride

“…The adhesion energies have been computed using different ab-initio and semi-empirical approaches 6,12,23,26 . These calculations give values in the range of some tens of meV per unit cell for ∆ AB , and much lower for ∆ BA .…”

Versatile van der Waals Density Functional Based on a Meta-Generalized Gradient Approximation

“…However, the M06-L has molecular systems in its construction, while SCAN is not fitted to any bonded system. Constrained by the 17 exact constraints and guided by a set of appropriate norms, the resulting mathematical form of SCAN achieves excellent control of error cancellation between semilocal exchange and correlation, not only for covalent bonds as LDA and GGA do, but also for the intermediate-range vdW interaction, although the resulting binding still comes from the exchange part instead of the correlation [44,45]. A similar situation occurs in covalent molecules, where semilocal exchange rather reliably imitates exact exchange plus leftright correlation [46,47].…”