I missed this short communication last year, (thanks to the computational chemistry list CCL for bringing this to our attention!) but it is worth commenting on even a year later as this topic is one that frequently confuses users.
Ho, Klamt, and Coote1 note that popular quantum chemistry codes, including the Gaussian series, present the output of continuum solvent models in a way that can be misleading. What is called the free energy is in fact the sum of the electronic energy in solution and the free energy associated with non-electrostatic contributions. What is missing are corrections to the solute to give its free energy. What is assumed (oftentimes without fully recognizing this assumption) is that the thermal corrections for the solute in the gas and solution phase will cancel – but this does not have to be. Let the QM code user (and reader of the literature) beware!
References
(1) Ho, J.; Klamt, A.; Coote, M. L., "Comment on the Correct Use of Continuum Solvent Models," J. Phys. Chem. A 2010, 114, 13442-13444, DOI: 10.1021/jp107136j
Henry Rzepa responded on 12 Jan 2012 at 2:04 am #
There has been a dialog between Klamt and Cramer on the CCL list about this. I think it fair to say that they agreed to disagree on 75% of the interpretations, and agreed on the other 25% So in the interests of balance, anyone interested in the topic should go read the CCL list as well as the Klamt article Steve reviews.
Olga Dmitrenko responded on 23 Jan 2012 at 1:21 pm #
Here is what I am writing in my current manuscript on this: “Klamt highlighted the shortcoming and confusing issues on the usage of continuum solvent models, and noted that many of the studies (ref.6 in [1]) that employ Gsolv=Esolv+ΔGcorr_solv to obtain solution-phase free energies “often deliver reasonably accurate results”. Consequently, we also considered the free energies of solvation given directly in the outputs of the solvent-phase optimizations….
Bottom line, I will be very much interested to hear new fresh opinions on this topic!
Thanks for mentioning the dialog on CCL. I will try to find it.
Chris Cramer responded on 09 Nov 2012 at 4:13 pm #
To update this conversation, an oft-debated point has been whether solute thermal contributions should derive from partition functions (sometimes referred to casually as “frequency calculations”) done for the gas phase or for the solvated structures. A detailed discussion of pros and cons is available at Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. “Use of Solution-Phase Vibrational Frequencies in Continuum Models for the Free Energy of Solvation” J. Phys. Chem. B 2011, 115, 14556 (doi:10.1021/jp205508z). Bottom line: solvated frequencies are just fine, and sometimes much more sensible a choice.