The adsorption of CH$_3$ and C$_6$H$_6$ on corundum-type sesquioxides: The role of van der Waals interactions

Van der Waals (vdW) interactions play an important role in the adsorption of atoms and molecules on the surface of solids. This role becomes more significant whenever the interaction between the adsorbate and surface is physisorption. Thanks to recent developments in density functional theory (DFT), we are now able to employ different vdW methods that helps us to account for the long-range vdW forces. However, the choice of the most efficient vdW functional for different materials is still an open question. In our study, we examine different vdW approaches to compute bulk and molecular adsorption properties of M$_2$O$_3$ oxides (M: Cr, Fe, and Al) as well-known examples of the corundum family. For the bulk properties, we compare our results for the heat of formation, cohesive energy, lattice parameters and bond distances as obtained using the different vdW functionals and available experimental data. Next we compute the adsorption energies of the benzene molecule (as an example of physisorption) and CH$_3$ (as an example of chemisorption) on top of the (0001) M-terminated and MO-terminated surfaces. Calculating the vdW contributions into the adsorption energies, we find that the vdW functionals play important role not just in the weak adsorptions but even in strong adsorption.