Subject: Reliability of the solvation models in GAUSSIAN

From: shochen michael <[email-masked]>
Date: Mon, 4 Aug 1997 14:28:24 +0300 (IDT)

Dear Netters

 In my current work I need to estimate oxidation potentials
for sevelal molecules. In other words, I need to calculate
the energy difference between ground state and cation-radical
(lowest doublet). I have experimental estimations of the oxidation
potentials for some of my compounds in water solution. All of my
molecules contain three sulfonic groups (SO3-), which of course are
ionized in water. Thus, I have as a ground state anionic species with
the charge -3, which do not exist in vacuum as systems with negative 
orbital enrgies of their HOMO (the oxidized form has the charge -2).
Vacuum calculations are irrelevant to my compounds, so I have used
a reaction field model implemented in GAUSSIAN-94 to simulate water
solution. I have used IPCM model with dielectric constant 80.
The level of quantum calculations was B3LYP/6-31G* (my molecules are very
big, so I can't use more extensive basis set with diffuse functions).
Unfortunately, for several cases I got very big (more than 1 eV)
difference between the calculated in my model and experimental values
of the oxidation potentials.
  My question is: How really are reliable the estimations of the solvation
effects in the IPCM model? Do somebody know references on examples
providing the comparison between theoretical and experimantal estimations
of solvation energies, or any physical values, which
are dependent on the solvation effects for the IPCM model ?

Thank you in advance.
Michael     
*********************************************************

Dr. Michael Shokhen
Senior researcher,
Department of Biochemistry,
G.S.Wise Faculty of Life Sciences,
Tel-Aviv University,
69978, Tel-Aviv, Israel.
Fax: 972-3-6415053
Email: [email-masked]

*********************************************************

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From: [email-masked] (Christopher Cramer)
Date: Tue, 5 Aug 1997 10:04:51 -0500 (CDT)

Michael,

Re: CCL post on reliability of IPCM models in G94 for trianion/dianion
    solvation free energies

I would expect IPCM to do very poorly indeed for a trianion. The philosophy
of IPCM is to define a continuum cavity boundary using an isodensity
surface, and the developers have done some work illustrating this
prescription to give "reasonable" cavities (there's no unique way to decide
what the cavity SHOULD be) for neutrals. In the case of polyanions,
however, one might have to go quite far from the molecule to reach the
required isodensity threshhold, in which case the cavity volume will be
too large and the solvation energy too small. You might be getting lucky,
however, since you aren't using diffuse functions, so the density can't
spread out too far.

A separate problem is the so-called charge penetration phenomenon. The PCM
approach, unlike generalized Born or multipole expansion, only accounts for
the charge density inside the continuum boundary. This leads to the
remarkably unphysical problem (if not corrected for -- as far as I know G94
applies no correction) that the solvation free energy of an anion begins to
DECREASE with decreasing cavity volume at some point because the charge
enclosed by the boundary is decreasing (the Born formula says solvation
free energy is proportional to the square of the charge and inversely
proportional to the cavity radius for a sphere, recall). So, in a trianion,
more density will be outside the cavity than for your dianion (say,
perhaps, leading to actual solvated charges of -2.8 and -1.86 or so)
and given the large electrostatic solvation free energies associated with
multiply charged systems, errors on the order of 1 eV would not be at all
surprising.

Tomasi has spent a lot of effort addressing the charge penetration problem
in the Pisa group's various implementations of PCM (which use fixed cavity
boundaries, incidentally, typically employing a union of spheres having
radii of atomic van der Waals radius times 1.2), and Klamt has recently
mixed ideas from multipole expansion into his COSMO modification of PCM in
order to deal with this too.

As for your question of benchmarking of the IPCM model against experiment,
such an effort would be difficult to carry out. The IPCM model accounts
only for the electrostatic component of solvation, but for neutral
molecules, non-electrostatic solvation components may be considerably
larger or of the same magnitude as the electrostatics, so errors would be
difficult to assign (the electrostatic component is not a physical
observable). For ions, on the other hand, the electrostatic component
should dominate, but now the experiment is rather hard to do, and there are
few data against which to compare (perhaps twenty-five or so solid aqueous
free energies of solvation and another fifteen with non-trivial error
bars--substantially fewer data in any individual organic solvent).

Chris
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-- 

Christopher J. Cramer
University of Minnesota
Department of Chemistry
207 Pleasant St. SE
Minneapolis, MN 55455-0431
--------------------------
Phone:  (612) 624-0859 || FAX:  (612) 626-2006
[email-masked]
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