Subject: Re: CCL:G:Charges

From: A J Turner <[email-masked]>
Date: Thu, 24 Oct 1996 15:10:02 +0100 (BST)

Hi!

I am working on the charge fitting area right now.  My experiance may be
of interest.  The approxiation of placing charges on nuclear centres can
greatly outweigh to variation in method of attaining the charges in the
first place.

I am using QM/MM technology to _directly_ fit a MM force field to a QM/MM
force field.  This shows that several 'off centre' points _must_
be included if one is to get anything like a reasonable fit of the mm
forces to the qm ones.  My present example being ch3o- , where the forcing
of the negative charg onto the 'centre' of the oxygen causes maiisve
'artificial' +ve charge build up on the carbon and hence large negaive
charges on the protons.  The use of a QM/MM force field where the same vdw
paramters apply shows that these negative protons are very very far from
correct.

Does anyon else have similar experiances?

Best wishes

Alex

 -------------------------------------------------------------------
|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
 -------------------------------------------------------------------