AMBER Archive (2005)

Subject: AMBER: RESP: Question

From: Carlos Jaime (carlos.jaime_at_uab.es)
Date: Mon Dec 05 2005 - 11:43:16 CST

Dear all,
Our group is using Amber since long ago. Nowadays we are sistematically
using Amber v.7, and one of my Ph.D student face with one problem with
RESP that I want to expose.
We are using RESP to get a set of atomic charges that are 'independent
from the conformation' of the substrate. We usually compute several
conformations with an ab initio program (usually Gaussian), get the
structure and energies, and we use the structure, the energy and the
atomic charges into RESP, with the multiconformer option, to get an
unique set of atomic charges.
When working with fluorine compunds, we realized that averaging two
conformers, the fluorine atoms got a charge almost zero. We checked the
behaviour of the RESP program with individual conformations and is
working properly, but as soon as we are using more than one conformer,
the charges are greatly reduced and several values are finally near zero.
We were really surprised and we started to search through the RESP code.
We found thet RESP do not average charges but average the molecular
electrostatic potentials. This means that the average of the meps of two
conformers that are enantiomeric is almost null, and atomic charges are
then also almost zero.

Finally we end up with two possibilities:
1.- we are using RESP in a wrong way
2.- something in RESP is not properly working

Has anyone faced with this problem before?
Can someone give us some information about this point?

Thanks in advance. 

-- 
Carlos JAIME
Dept. Quimica, Fac. Ciencias (Cn)
Univ.Autonoma Barcelona
08193 Bellaterra (Barcelona), SPAIN
Phone: ++34-935812591
Fax: ++34-935811265 or ++34-935812477

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