Subject: Fitting point charges to ESP with Spartan

From: Bjoern Rabenstein <[email-masked]>
Date: Fri, 01 Nov 1996 11:46:17 +0100

Hi!

Does anybody know the exact method used by Spartan to calculate point
charges by fitting to the electrostatic potential? Or at least a
reference to relevant papers?

Nothing is written about that in the manual (except some very general
information about the fitting procedure). The program tells only the
number of points used.

I did a calculation with Gaussian using CHELPG and got similar, but not
identical results as with Spartan.

In a book about using Spartan I found a reference to the Breneman &
Wiberg-paper (Journal of Comp. Chem, 11, pp. 361-373 (1990), but I'm not
very sure that Spartan is using exact the method described in the paper.

Thanks for help!

-- 
     Bjoern Rabenstein * Freie Universitaet Berlin * Inst. f. Biochemie
 Inst. f. Kristallographie * AG Knapp * Takustrasse 6  * D-14195 Berlin
                 [Telephon] +49-30-838-3484   [Telefax] +49-30-838-3464
 [email] [email-masked]  [WWW] http://www.chemie.fu-berlin.de/~rabe/

---

From: "Wayne Huang" <[email-masked]>
Date: Fri, 1 Nov 1996 14:46:02 -0800

On Nov 1, 11:46am, Bjoern Rabenstein wrote:
> Subject: CCL:G:Fitting point charges to ESP with Spartan
> Hi!
>

Spartan does use CHELPG for electrostatic fit charge partition. See:
C. Brenneman, K.B. Wiberg, J. Comput. Chem., 11, 361-373 (1990)
L.E. Chirlian M.M. Franci, J. Comput. CHem.,  8, 894     (1987)

As there is discrepancy on resulting charges from G9x and Spartan, that
would not be surprising. Although based on the same idea, the implementation
could be different. It is somewhat arbitrary in at least two ways, the
number of grid points chosen and the how far the region included. In plain
English, it basically paints a layer of grease on the vdw surface. Too far
beyond the vdw surface would get the charges too small (eventually approach
zero). The extent of the exclusion of the inner volume will also affect the
outcome (too close to the centers results in charges being too large). I would
tend to believe the two implementations should differ by a factor, may or may
not be linearly scalable.

In the nutshell, regardless which partition scheme you use, stick with it
systematically which should give you sensible comparison (hopefully).

Have a good one!

--Wayne

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