From chpajt@bath.ac.uk Thu Oct 24 10:31:44 1996 Received: from goggins.bath.ac.uk for chpajt@bath.ac.uk by www.ccl.net (8.8.0/950822.1) id KAA23906; Thu, 24 Oct 1996 10:23:51 -0400 (EDT) Received: from bath.ac.uk (actually host mary.bath.ac.uk) by goggins.bath.ac.uk with SMTP (PP); Thu, 24 Oct 1996 15:10:26 +0100 Date: Thu, 24 Oct 1996 15:10:02 +0100 (BST) From: A J Turner To: "Schiffer, Heinz, Dr., WIA / SC" cc: CCL Subject: Re: CCL:G:Charges In-Reply-To: <004DAC30.MAI*/S=Schiffer/OU=MSMWIA/O=HOECHST/PRMD=hoechst-ag/ADMD=dbp/C=de/@MHS> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII 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 |A.J.Turner@bath.ac.uk | |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 | -------------------------------------------------------------------