CCL: Empirical Dispersion Correction

From: "Stefan Grimme" <grimme**thch.uni-bonn.de>
Subject: CCL: Empirical Dispersion Correction
Date: Thu, 27 Aug 2015 06:05:22 -0400

 Sent to CCL by: "Stefan  Grimme" [grimme##thch.uni-bonn.de]
 Dear Adedapo,
 dispersion corrections to B3LYP are essential even in "non-obvious"
 cases because they correct for fundamental correlation energy deficiencies
 in the functional at medium- as well as long interatomic distances.
 This is not directly related to the issue of explicit solvation.
 In order to properly compute protonation constants (I assume a free
 energy property) its essential that you inclcude ALL free energy
 contributions (internal as well as solvation) accurately.
 In most (electronically simple) cases when B3LYP is better than
 B3LYP-D3 there is a hidden
 error compensation and the good result you get is based on the
 wrong reason. In summary: forget B3LYP, use always B3LYP-D3 or other
 dispersion corrected versions of B3LYP (or even more modern
 dispersion-inclusive functionals).
 Hope this helps
 Stefan