Thursday, March 26, 2015

Second reviews of the PCCP paper - almost there

PCCP writes: "I am pleased to inform you that your revised manuscript has been recommended for publication in Physical Chemistry Chemical Physics subject to further revision in line with the attached reports."

Referee: 1
Comments to the Author
Most of my comments have been addressed.

However, I am still concerned about the recommendations for treating low frequency vibrations.  The harmonic oscillator approximation is not appropriate for low frequencies because the entropy term goes to infinity.  As recommended by Grimme (2012), a hindered / free rotor may be a better treatment (for related methods and discussion see TRUHLAR, DG, J COMP CHEM 1991, 12, 266-270 DOI: 10.1002/jcc.540120217 and McClurg, RB, Flagan, RC, Goddard, WA JCP 1997, 106, 6675-6680 DOI: 10.1063/1.473664)

My comment about explicit water molecules also concerned entropy.  While including explicit water molecules may increase the CPU time, this can be overcome.  What extra CPU time cannot overcome is the fact that the explicit waters in reality can exchange with the bulk water increasing their entropy compared to a RRHO calculation of their entropy.  It is best to include explicitly only tightly bound

Immediate reactions
Point 1:
* The paper lists options for how to treat low frequency vibrations but there is not a specific recommendation because the issue as it applies to binding free energies has not been thoroughly compared.
* Grimme's approach is not derived from first principles and is therefore not a priori better than other approaches.  It will in some cases treat modes that are clearly stretch vibration as a free rotor
* The free rotor entropy also goes to infinity as the frequency approaches zero.

Point 2:
* Any such effect is included implicitly in the parameterization of the solvation free energy of H2O.
* Any error in this parameterization is largely cancelled by using the water cluster approach recommended in the paper.
* Bryantsev et al. have shown that using the water cluster approach leads to smooth convergence of solvation free energies of H+ and Cu2+ that are in good agreement with experiment.

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