Van der Waals Interactions
van der Waals (dispersion) interactions are weak, long-range attractions arising from correlated electron fluctuations. Local and semilocal density functionals (LDA, GGA) do not describe them correctly, so they are added through empirical pairwise corrections (such as DFT-D) or through nonlocal correlation functionals (such as vdW-DF and BEEF-vdW). They matter for physisorption, layered materials, molecular crystals and densely covered surfaces.
Coverage-dependent vdW contributions to atomic adsorption (2016)
Using the nonlocal correlation term of BEEF-vdW as a measure of the vdW part of the adsorption energy, Thirumalai and Kitchin found that for atomic adsorbates on Pt(111) and Pd(111) this contribution becomes more attractive as coverage increases, and is larger for big, polarizable atoms (S, Cl, Br) than for small ones (F smallest) (Thirumalai 2016). Hollow and atop sites gave nearly the same vdW magnitudes, suggesting it depends mainly on the adsorbate, and it correlates with effective nuclear charge. Counterintuitively, the vdW-inclusive functional binds the adsorbates more weakly than PBE; the authors explain this with effective medium arguments, in which attraction to neighboring adsorbates reduces the electron density each atom needs from the surface. Summary: thirumalai2016.