CCL: PCM vs. explicit water with PCM - effect on reaction profile

 Sent to CCL by: Marcin Krol []
 Dear Gonzalo,
 The problem of adding explicit solvent molecules to continuum solvation
 models was recently discussed in:
 Kelly etal, JCTC 1:1133-1152, 2005
 The authors were able to get better solvation energy estimate if they
 added one explicit solvent molecule to the solute and performed implict
 solvent calculations on such system using their SM6 model. However, for
 other implicit solvation models adding an explict solvent molecule made
 the results worse (which goes in line with what you wrote).
 See also papers by JR Pliego
 JPCA 105:7241, 2001; JPCA 106:7434, 2002
 where he developed and tested mixed implict-explict solvent models.
 Hope this helps
 On Tue, 10 Oct 2006, Gonzalo Jimenez Oses gonzalo.jimenez*_*
 > Sent to CCL by: "Gonzalo Jimenez Oses"
 > Dear CCL'ers
 > I am also very interesting in the differences emerging from introducing
 > one or more explicit solvent molecules in PCM calculations (or any other
 > continuum models). I also asked something related to this question, and
 > probably the most valuable answer I obtained was offered by Dr. Frank
 > Jensen (thank you!):
 > "One thing to consider is that PCM type models are parameterized
 > and the values of the parameters have been derived to fit experiments,
 > _without_ an explicit first solvation shell. A large fraction of
 > solvation is the first shell, and that has been absorbed by the
 > parameters. Introducing an explicit solvation shell and then put a PCM
 > on top of that may therefore give you a kind of double counting. Of
 > course the cavity changes, and there are the usual problems of electron
 > density outside the cavity, etc.... In short, there is a risk that the
 > results will _deteriorate_ when putting in an explicit first solvation
 > shell. I am not aware of any study where the performance of PCM type
 > models where the first solvation shell is considered explicitly, but if
 > you find any, I would appreciate the reference(s). In principle such an
 > approach should be able to provide more accurate results than a pure
 > continuum model, but the PCM parameters may have to be returned. This,
 > of course, is not trivial, and the optimal set of parameters may depend
 > on which method you use for the solvent molecules (HF, DFT, MP2, etc).
 > Sorting this out should keep you busy for a while ;-)"
 > I hope this could help a little. In my experience, this kind of
 > discrepancies are much more accusated in the case of charged molecules,
 > in which the inclusion of solvent effects are usually needed. When
 > possible, a good alternative should include one or more counterions to
 > neutralize the charges.
 > In any case, I recommend you to read the interesting posts coming from
 > Prof. Andreas Klamt (COSMO model), which is an expert on this field.
 > Best regards,
 > Gonzalo