AMBER Archive (2004)

Subject: Re: AMBER: RESP vs. AM1-BCC charges for isoprene

From: Oliver Hucke (ohucke_at_u.washington.edu)
Date: Fri Jul 23 2004 - 12:41:05 CDT

Hi Araz,

thanks very much for your helpful reply.
Of course you are right saying "It does not make sense to design a
charge model based on another charge model."
What I actually meant was that the AM1-BCC method was designed to
reproduce the HF/6-31G* esp as the resp fit procedure was. Sorry for not
being precise.

I would like to add the following questions:

o Is there a way to quickly check if the training set used for the
AM1-BCC parameterization is suitable for a specific molecule?

o Is there a publicly accessible repository for the most recent
parameterization of the AM1-BCC method (In your paper you wrote that you
had begun the global reparamaterization of the charge model).
Or is the globally parameterized method already part of the amber8
distribution?

Best regards,
Oliver

ajakalian_at_lav.Boehringer-Ingelheim.com wrote:
> Hello Oliver,
>
> AM1-BCC was NOT designed to reproduce RESP charges.
>
> It does not make sense to design a charge model based on another charge
> model. We designed the AM1-BCC model to reproduce the QM electrostatic
> potential (ESP) at the HF/6-31G* level of theory. This is the same
> fundamental idea behind RESP, however unlike RESP, AM1-BCC does not use
> HF level of theory but rather the AM1 level to get a first approximation
> of the QM ESP with the Coulson charges. These first-pass charges are
> then corrected with bond-charge corrections (BCCs) in order to reproduce
> the QM ESP.
>
> That said, I would not worry about large differences between AM1-BCC and
> RESP for carbon atoms. It is well known that RESP charges suffer from
> numerical instabilities, especially for buried atoms (i.e. charges can
> vary widely while the RMS of the fit varies little). Table VI in the
> AM1-BCC method paper shows this perfectly. Look at the RESP charges on
> Csp3 atoms, they vary between -0.08 and +0.07, while for the same atoms
> the AM1-BCC charges vary between +0.09 to +0.1.
>
> Hope this helps,
>
> Araz
>
> ****************************
> Araz Jakalian, Ph.D.
> Research Scientist, Structural Research Group
> Boehringer Ingelheim (Canada) Ltd.
> Research and Development
> 2100 Rue Cunard
> Laval, Quebec, H7S 2G5, Canada
> Phone: (450) 682-4640 Fax: (450) 682-4189
> E-mail: ajakalian_at_lav.boehringer-ingelheim.com
> ****************************
> 

-- 
_______________________________________________________________

Oliver Hucke, Dr.
                               Health Sciences Building - K418C
University of Washington      1959 NE Pacific St.
Dept. of Biochemistry         phone: (206) 685 7046
Box 357742                    fax  : (206) 685 7002
Seattle, WA 98195-7742        email: ohucke_at_u.washington.edu
_______________________________________________________________

-----------------------------------------------------------------------
The AMBER Mail Reflector
To post, send mail to amber_at_scripps.edu
To unsubscribe, send "unsubscribe amber" to majordomo_at_scripps.edu