I wanted to make a small resume and add commentaries on this entropy of solvation topic. Some people (Purisima, Gilson) accurately state that the volume of the box is the same regardless of having or not a solvent in it, so ideally there should be no loss of translational entropy. I agree, although some claim that the accesible volume is reduced by the solvent. Others (Ribeiro) say that the partition function in a solvent is way to different to the Sackur-Tetrode gas phase translational entropy. I also agree. My physical chemistry university teachers told us that the solvent is reorganized around a solute molecule, and this means a loss of entropy. This effect should be considered when calculating the solvation free energy. Acording to this we should use gas phase free energy and add the solvation free energy.

Sadly in most programs this doesn't work. As already posted the best values I could find are between gas phase energy and free energy. The solvent models are not good enough to account for this loss of entropy. This is clear when trying solvation at different temperatures, where the solvation free energy remains unchanged, something contrary to the free energy concept. Only molecular dynamic calculations take this into account, but this is impossible to achieve for most of quantum chemistry mortals like me.

Klamt argues that COSMO-RS resolves this problem, but I have never worked with this model so I cannot comment. I'd love to see referencees or accurate data on this, but I got lost on the COSMO web page and couldn't find this information. If someone knows a paper that use this program and clearly details the solvation, please write down the cite.

I want to add some referencees I could find on this, in case someone is interested.
Theoretical works: (a) Tomasi, J; Persico, M. Chem. Rev. 1994, 94, 2027. (b) Tomasi, J. Theor. Chem. Acc. 2004, 112, 184. (c) Ben-Naim, A. J. Phys. Chem. 1978, 82, 792.
Practical works: (a) Tamura, H.; Yamazaki, H.; Sato, H.; Sakaki, S. J. Am. Chem. Soc. 2003, 125, 16114. (b) Sakaki, S.; Takayama, T.; Sumimoto, M.; Sugimoto, M. J. Am. Chem. Soc. 2004, 126, 3332. (c) Sumimoto, M.; Iwane, N.; Takahama, T.; Sakaki, S. J. Am. Chem. Soc. 2004, 126, 10457. (d) Braga, A.; Ujaque, G.; Maseras, F. Organometallics 2006, 25, 3647.

I hope I'm not making mistakes on all this issues. I'm not a specialist, I just want to be practical and as accurate as possible on my calculations.
Finally I want to thank Luis Simon for starting this discussion. I feel I'm not alone in my doubts.

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..........Sebastian Kozuch...........
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...The Lise Meitner-Minerva Center...
.for Computational Quantum Chemistry.
...Hebrew University of Jerusalem....
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http://yfaat.ch.huji.ac.il/kozuch.htm
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