AMBER: RESP charge fitting for N-methylguanine cation

From: Grzegorz Jezierski <>
Date: Thu, 12 May 2005 18:50:17 +0200

Dear AMBER users,

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:

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?). 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*.

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

Thank you for help.
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Received on Thu May 12 2005 - 18:53:01 PDT
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