From owner-chemistry-: at :-ccl.net Mon Dec  8 13:59:01 2008
From: "Raphael Ribeiro raphaelri(-)hotmail.com" <owner-chemistry:server.ccl.net>
To: CCL
Subject: CCL: translational entropy in solvent
Message-Id: <-38262-081208135819-1181-8kYPQipGRdZwBT9ElcxU0A:server.ccl.net>
X-Original-From: "Raphael  Ribeiro" <raphaelri(a)hotmail.com>
Date: Mon, 8 Dec 2008 13:58:16 -0500


Sent to CCL by: "Raphael  Ribeiro" [raphaelri%%hotmail.com]
Dear Pierre,

It is quite obvious that the translational entropy changes while going from the ideal gas phase to a solvated phase. What is also obvious in my opinion is that the translational/rotational/vibrational components of free energy (and also entropy) calculated using implicit solvents are not going to reproduce the quantities from the real system in a good way. I've said earlier that one of the biggest failures of the implicit solvent models is that they do not represent in a true way the phase space of the system. The density of states calculated using the implicit model for solvation does not represent the reality at all, and that is why it is not a good idea to calculate components of the free energy with PCM, COSMO, etc.

Also, in most of the implicit models there are empirical parameters (in the non-electrostatic contributions) which are not physical. Implicit solvation models were created to give nice solvation free energies (as a whole) and that is why they are in most of the times not suitable to reproduce other physical properties in a good way.

For your association/dissociation problem I recommend you the following article: http://biophysics.med.jhmi.edu/amzel/people/siebert/strsl_2col_bw_letter.pdf

In this article the authors create a model for calculating the loss of translational entropy in associations accurately. They use molecular dynamics.

The last thing to say is that the free energy of solvation generated by implicit solvent models already include all of the components of the free energy (free energy of solvation  = free energy of the solvated phase system - free energy of the gas phase system) and as Andreas Klamt already remarked, the addition of translational, rotational and vibrational free energy contributions in solutions will lead to double counting of effects already implicitly taken into account in solvation models and are not recommended at all.

Raphael Ribeiro