A peculiarity in the coronene dimer results is that the def2-SVPD error is significantly larger (at 15 kcal/mol) than the def2-SVP error (8 kcal/mol). While CP correction reduces both errors to 3 kcal/mol, the larger error when diffuse functions are added defies the conventional wisdom that a good description of the tails of the wave function is important to the description of non-covalent interactions. In fact, larger errors for def2-SVPD than for def2-SVP are a feature of the dispersion-bound subset of S66, for every functional that we have examined (see Tables ), although the overall S66 convergence errors are reduced in the presence of diffuse functions, primarily due to a reduction in convergence errors for the hydrogen-bonded subset of S66. (We have observed before that diffuse functions are particularly important for the description of hydrogen bonds. ) For the dispersionbound complexes in S66, results with def2-SVP lead to overbinding (for all functionals examined), and since CP correction generally reduces interaction energies, these results benefit from some error cancellation that disappears when diffuse functions are added, leading to less overbinding with def2-SVPD. This type of error cancellation feels fragile and thus from our point of view, CPcorrected results obtained using def2-SVPD is a more reliable representation of the true performance of double-ζ basis sets.