For those of you interested in learning about dispersion corrections for density functional theory, I recommend Grimme’s latest review article.1 He discusses four different approaches to dealing with dispersion: (a) vdW-DF methods whereby a non-local dispersion term is included explicitly in the functional, (b) parameterized functional which account for some dispersion (like the M06-2x functional), (c) semiclassical corrections, labeled typically as DFT-D, which add an atom-pair term that typically has an r-6 form, and (d) one-electron corrections.
The heart of the review is the comparison of the effect of including dispersion on thermochemistry. Grimme nicely points out that reaction energies and activation barriers typically are predicted with errors of 6-8 kcal mol-1 with conventional DFT, and these errors are reduced by up to 1.5 kcal mol-1 with the inclusion fo the “-D3” correction. Even double hybrid methods, whose mean errors are much smaller (about 3 kcal mol-1), can be improved by over 0.5 kcal mol-1 with the inclusion of the “-D3” correction. The same is also true for conformational energies.
Since the added expense of including the “-D3” correction is small, there is really no good reason for not including it routinely in all types of computations.
(As an aside, the article cited here is available for free through the end of this year. This new journal WIREs Computational Molecular Science has many review articles that will be of interest to readers of this blog.)
(1) Grimme, S., "Density functional theory with London dispersion corrections," WIREs Comput. Mol. Sci., 2011, 1, 211-228, DOI: 10.1002/wcms.30