For revPBE and RPBE, the SD motif only becomes feasible when you switch from GD2 to GD3. Again, this makes sense based on the numbers in Table : SD has the largest change in magnitude (and becomes more negative) compared to LD and LC. While the Grimme methods use a semi-empirical dispersion correction, there are other ways to account for the van der Waals interaction. One prominent method is to use a fully non-local functional, like vdW-DF2. In this formulation, the exchange correlation energy takes the form