CCL: translational entropy in solvent

 Sent to CCL by: "Raphael  Ribeiro" [raphaelri*|*hotmail.com]
 Quantum chemistry packages usually calculate translational entropy and the other
 thermal effects using the ideal gas approximation, even if you do the
 optimization-hessian task using a solvent model.
 In general, the free solvation energy calculated in implicit solvent models
 accounts only for polarization effects. It may be calculated as the work done in
 isothermal conditions to polarize(or charge) the system, or as an electrostatic
 potential energy. Some of the implicit models also include non-eletrostatic
 contributions (cavitation, dispersion and structure effects). This free energy
 of solvation is defined as the difference between the free energy of the system
 in the solvated phase and in the gas phase.
 It is not correct to add  translational entropy in the free energy of solvation.
 The free energy of solvation, if calculated accurately, should include the
 difference between the translational entropy of the two phases and also all of
 the other components of entropy and enthalpy. The only problem is that in most
 of the cases you can't calculate it in an accurate way. And the reason is quite
 obvious, the solvent in the real world isn't  a continuous medium.
 The procedures you outlined below are in my view not correct. The
 components(translational,rotational,etc) of enthalpy and especially entropy of a
 system calculated in solvated phases using implicit solvents are not accurate at
 all, as your phase space is very limited (most of the phase space comes from the
 solvent and a continuous solvent does not have any degree of freedom). So you
 have to be very careful when using this kind of information.
 Implicit solvent models were made in a way the best we can get from them is the
 free energy of solvation, and some of them are even parametrized to give precise
 free energy of solvations. Any other information given when you use these models
 (components of entropy,etc) in most of the times does not mean anything.
 Raphael Ribeiro
 > From: owner-chemistry!A!ccl.net
 > To: raphaelri!A!hotmail.com
 > Subject: CCL: translational entropy in solvent
 > Date: Wed, 3 Dec 2008 11:08:31 -0500
 >
 >
 > Sent to CCL by: "Luis M Sim n" [lsimon|-|usal.es]
 > Many quantum chemistry packages include implicit solvation models, such as
 PCM, CPCM, COSMO,
 > etc. Nevertheless, even when the optimization was done using any of those
 solvation models, I have
 > observed that the thermal contributions to delta G are calculated assuming
 that the molecules are in
 > the gas phase. Therefore, the translational entropy is overestimated (see,
 for example, C. Hunter,
 > ACIE, 43, 5310-5324, 2004).
 >
 > There are models that can estimate this translational entropy in solution
 (Warshell et al.,
 > J.Phys.Chem.B, 104, 4578-4584, 2000), but unfortunately these are not
 straightforward to implement,
 > and computational requirements might exceed our possibilities.
 >
 > On the other side, I have seen that implicit solvent calculations many
 times offers a "free solvation
 > energy". I do not know if, in that free solvation energy, the
 translational energy is, somehow,
 > evaluated and included, or if it just accounts for cavitation and
 polarization contributions.
 >
 > My question is: is it correct to add up the translational estropy
 calculated assuming gas phase
 > behaviour to that free solvation energy? Should not it be more correct if
 the vibrational or even
 > rotational contributions are included in deltaG calculations but
 translational contribution is excluded?
 > Does anyone knows any easy method for, approximately, account for the
 translational entropy in
 > solution, in case that have to be added to the vibrational and rotational
 contributions?
 >
 > Sorry if that issue have previously been discussed.