CCL: translational entropy in solvent
- From: "Raphael Ribeiro"
- Subject: CCL: translational entropy in solvent
- Date: Mon, 8 Dec 2008 13:58:16 -0500
Sent to CCL by: "Raphael Ribeiro" [raphaelri%%hotmail.com]
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:
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.