CCL: Empirical Dispersion Correction

Dear Stefan,
                      Thanks a lot for your explanation. Could you please
 shed more light on the likely sources (technical or personal) of this
 hidden error compensation that could have led to my obtaining good results
 with B3LYP as against that of B3LYP-D3? And is there any step I can take
 computationally to correct for this error or this is just system dependent?
 Once again, thank you very much for your help.
 On 27 August 2015 at 12:05, Stefan Grimme grimme : thch.uni-bonn.de <
 owner-chemistry.:.ccl.net> wrote:
 >
 > 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>
 >
 >
 --
 Adedapo Adeyinka
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