From owner-chemistry@ccl.net Thu Aug 27 07:36:01 2015
From: "Stefan Grimme grimme : thch.uni-bonn.de" <owner-chemistry,,server.ccl.net>
To: CCL
Subject: CCL: Empirical Dispersion Correction
Message-Id: <-51626-150827060524-13640-+soOQL+WlkHqDur064wvRA,,server.ccl.net>
X-Original-From: "Stefan  Grimme" <grimme**thch.uni-bonn.de>
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