From chemistry-request |-at-| ccl.net Fri Jan 16 13:45:11 2004
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Date: Fri, 16 Jan 2004 13:45:01 -0500
From: "Curt M. Breneman" <brenec~at~rpi.edu>
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To: FyD <fyd~at~u-picardie.fr>
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Subject: Re: CCL:CHELPG in GAMESS
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Dear Francois,

I'm not familiar with the specific CHELPG modifications made for GAMESS, but one
issue with the original CHELPG routine was the possibility of computing atomic
charges that do not coincide with molecular symmetry.  In the original
implementation, no special attention was paid to assuring that the 3D  grid of
sampled points was symmetrically distributed with respect to molecular symmetry
elements.  The effect can result in unequal ESP charges being computed for
symmetrically-related atoms.   This is not always a problem - In fact, it was found
that for small molecules using dense grids, (0.2 au or so), this was a minor effect.
In practical applications, this can raise an issue. The variation can be large or
small, depending upon the complexity of the electrostatic field and the sparsity of
the sampling grid.

Cheers,

Curt Breneman
RPI Chemistry


FyD wrote:

> Dear All,
>
> I am trying to use CHELPG in GAMESS to get electrostatic potential. In the
> GAMESS doc., it is written:
> "CHELPG: to use a modified version of the CHELPG algorithm, which produces a
> symmetric grid of points for a symmetric molecule"
>
> - Does it mean that the CHELPG algorithm in GAMESS is different from the
> published one in Breneman & Wiberg, J Comput Chem 1990, 11, 361-373 ?
> - If yes, why this algo. has been modified in GAMESS and/or is there a way to use
> the original one in GAMESS ?
>
> Thanks, regards, Francois
>
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