AMBER Archive (2008)

Subject: Re: AMBER: Question about resp charges for a charged molecule...

From: Germain Vallverdu (germain.vallverdu_at_lcp.u-psud.fr)
Date: Fri Oct 10 2008 - 06:20:45 CDT

Hello Alberto

Effectively, a partial charge of +5 is not really acceptable. Is this a
large molecule ?

The main problem when you compute partial charges is the fact that you
have a lot of parameter (the charges) and then you have a lot of local
minimum which give the same RMS or RRMS. A way to reduce the number of
parameter, and obtain charges more acceptable is to use constraint on
the charge. For example you could use symmetry of the system.

An other way is to use RESP charges insted of ESP charges. In the RESP
approach you search the smallest charges which fit well your potential
(you add a zero target on each charge). Maybe this could fix you problem
but it might not be implemented neither in Gamess or gaussian but some
codes should be available and are not difficult to do.

But you should also check your calculation because it is really strange.

Germain

PS : you can see for exemple
http://www3.interscience.wiley.com/journal/45001708/abstract?CRETRY=1&SRETRY=0

Alberto Sergio Garay a écrit :
> Dear all
>
> My name is Sergio Garay and I have some doubt about some of my
> results. I have obtained the resp charges of my molecule using gamess'
> ESP grid. I employed 6-31G* basis set in HF calculus. My molecule is
> charged +1 and I have obtained partial charges on some of the atoms
> very large. For instance, I have a +5 charge on one of my atoms. Is
> this reasonably? Could anyone give me any clue about how to
> parametrize the charge of a molecule as mine (molecule with a positive
> charge).
>
> Thank you in advance.
> 

-- 
Germain Vallverdu
Laboratoire de Chimie Physique
Université Paris Sud 11
germain.vallverdu_at_lcp.u-psud.fr
ma page perso <http://pagesperso.lcp.u-psud.fr/vallverdu/>
01 69 15 30 38 / 06 88 59 08 87
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