AMBER Archive (2003)

Subject: Re: AMBER: charge fitting

From: FyD (fyd_at_u-picardie.fr)
Date: Tue Nov 25 2003 - 10:42:56 CST

Dear Itzi,

> I am trying to calculate the charges for one organic molecule wich
> looks like this:
>
> H H
> | |
> O H O C = C
> || | | / \
> H-O(*)-C-C-N-S-C C-H
> | | \\ //
> H O C - C CH3
> / \ /
> H-C C-N
> \\ // \
> C - C CH3
> | |
> H H

ouf... difficult to understand...

> I am using the RESP metodology with 5 different conformations. I make
> charge restriction for the H in the carboxilic group, and I set
> multiple molecule atom equivalencing for the rest of the molecule atoms
> I find that the charge on the O(*) of the carboxilic group is
> extremely
> low ( -0.086) compared to that obtained with DFT calculations (-0.732)
> and with that one obtained with antechamber for a single conformation
> (-0.683)
> Can someone explain me why I get such a small charge when I use the
> RESP metodology?

Are you sure of your RESP inputs and MEP file, I mean, the atom order has to
be the same in all the conformations.

> And is there any way to use different conformations to create the
> file.prep with antechamber in a similar way that it is done using the
> RESP metodology?

Our prog. RED http://www.u-picardie.fr/labo/lbpd/RED/ does multi-orientation
RESP fit automatically (The RESP/GAMESS or GAUSSIAN inputs are automatically
generated). As for conformations, the different orientations has to present
the same atom order (in the MEP file and RESP inputs). RED does NOT perform
multi-conformation RESP fit yet, but, based on the multi-orientation inputs,
it should be straightforward to apply multi-conformation RESP fit. Running RED
should allows you to see where could be your problem by comparing the
different inputs...

I hope to release RED pretty soon...

Best regards, Francois

F.-Y. Dupradeau
 --
The Scripps Research Institute, San Diego, CA
Faculte de Pharmacie, UPJV, Amiens, France
 --
http://www.u-picardie.fr/labo/lbpd/fyd.htm

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