Re: AMBER: RESP charge fitting for N-methylguanine cation

From: FyD <>
Date: Fri, 13 May 2005 17:33:05 +0200

> As a result of a charge fitting procedure using RESP, performed for a
> cationic form (1+) of 9N-methylguanine, I could see that calculated
> point charges strongly depend on a basis set used and the density of MEP
> points specified in Gaussian input, namely:

It has been reported many times.

> 1) Increasing the 'density of points' parameter "IOp 6/42" from 1 to 6
> changed the optimized RESP charges drastically, about 0.1 e on some
> atoms. Further increase of the parameter from 6 to 8 gave the changes
> 0.05 e or lower, and in the same direction (convergence?).

It has been reported many times.

> Should I go
> up with the parameter? What's the precise definition of IOp 6/42? Is it
> in 1/(bohr sq.)? The Gaussian manual says "density of points per unit
> area in ESP fit" - does this mean the area of one of MEP layers? I kept
> all the time the default value for IOp 6/41 (=4). Both geometry
> optmization and population analysis was calc. with HF/6-31G*.

If you want to derive RESP charges for the Cornell et al. FF, just use R.E.D.
version I (or II):
please see:
for instance:

search using RESP ESP charge Derivation with google .
selecting mailing list mailing list archives

See also the Cieplak et al J. Comput. Chem 1995, 16, 1357-1377

> 2) I then changed the basis set to HF/cc-pVTZ and kept: IOp 6/41=4; IOp
> 6/42=8. Charges differed (between the two basis sets) by at most 0.05 e,
> except for the amino group, where the discrepancy was 0.1 e. Generally,
> one see more polarized charge distr. in the case of 6-31G*. Which basis
> set is more suitable as regards modeling polarization effects on the
> solvent?

It depends for which force field you want to derive the RESP charges:
The Cornell et al FF use 6-31G* basis set in the MEP computation which 'build
in' implicit polarisation in the charge values. Otherwise, you can use a basis
set which will generate MEP more in agreement with gaz phase, I guess...

You should find information about this in AMBER manual also...

Regards, Francois

  F.-Y. Dupradeau
DMAG EA 3901 & Faculte de Pharmacie, Amiens, France
The Scripps Research Institute, La Jolla, CA, USA
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Received on Fri May 13 2005 - 16:53:01 PDT
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