From owner-chemistry@ccl.net Sat Aug 27 11:15:18 2005
From: "CCL" <owner-chemistry|"|server.ccl.net>
To: CCL
Subject: CCL: CCL question about limitations of PCM
Message-Id: <-29025-050827065336-28598-Euty0K+hMlIb54fk8TN1yA|"|server.ccl.net>
X-Original-From: =?ISO-8859-1?B?Sm9z6SBJZ25hY2lvIA==?==?ISO-8859-1?B?R2FyY+1hIExhdXJlaXJv?= <jig|"|unizar.es>
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=ISO-8859-1
MIME-Version: 1.0


Sent to CCL by: =?ISO-8859-1?B?Sm9z6SBJZ25hY2lvIA==?==?ISO-8859-1?B?R2FyY+1hIExhdXJlaXJv?= [jig|"|unizar.es]
> We are investigating a transition state of a proton transfer mechanism.  We 
> successfully modelled such a state in the gas phase and are now looking at
> the 
> solvation effects using the PCM method.  However, the transfering hydrogen is
> 
> unbound (according to the gas phase) and upon trying to do a solvation 
> calculation we receive the following error.
> 
>  United Atom Topological Model (UA0  parameters set).
>  UA0: Hydrogen   21 is unbound. Keep it explicit at all point on the
>  UA0: potential energy surface to get meaningful results.
> 
> Is this error a limitation of the PCM or does anyone have advice on how to 
> correct this error?  Any assistance would be greatly appreaciated.

The problem lies in the use of United Atom radii to describe the solute cavity.
In the UA approach, heavy atoms and the hydrogen atoms bonded to them are
considered as a single unit to construct the solute cavity inthe SCRF model.
Try with one of the methods treating hydrogen atoms explicitly (UFF, Pauling,
Bondi), through the Radii=X keyword (more details in the Gaussian 03 manual,
SCRF keyword or in http://www.gaussian.com/g_ur/k_scrf.htm).
Hope this helps you.
Regards.

Jose I. Garcia