From arno@almaak.usc.edu Thu May 21 09:51:31 1998 Received: from almaak.usc.edu (almaak.usc.edu [128.125.7.166]) by www.ccl.net (8.8.3/8.8.6/OSC/CCL 1.0) with ESMTP id JAA01303 Thu, 21 May 1998 09:51:30 -0400 (EDT) Received: from localhost (arno@localhost) by almaak.usc.edu (8.8.8/8.8.8/usc) with SMTP id GAA10080; Thu, 21 May 1998 06:51:29 -0700 (PDT) Date: Thu, 21 May 1998 06:51:29 -0700 (PDT) From: Arno Papazyan To: "Donald E. Williams" cc: CHEMISTRY@www.ccl.net Subject: Re: CCL:Electrostatic model for ordinary hydrogen bonds In-Reply-To: <9805191443.ZM12041@xray5.chem.louisville.edu> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Heh, the "the disappearing hydrogen" is a fine technical point. However, the electrostatic effect of the HB would be a nice, round zero without the electrostatic parameters. **That** is the important effect of the HB. The disappearing hydrogen can be more fairly described as part of the force-field model, and is indeed a good way of taking the covalent part of the HB into account. Just like SHAKE is a good way of acknowledging that bonds exist between atoms without actually doing its QM. Regards, Arno Papazyan On Tue, 19 May 1998, Donald E. Williams wrote: > > There seems to be a widespread misunderstanding that an ordinary > hydrogen bond (say O-H...O) can be modeled with a purely electrostatic > potential by "dumping everything into electrostatic parameters". If anyone > knows of such a model, I would like to hear about it. > There is a parameterization called the "disappearing hydrogen" model of > the OHB. In this model, electrostatics are included, but also (and very > importantly) the repulsion of the hydrogen atom is set to zero, quite a drastic > change from a normal hydrogen atom. It is easily shown that the effect of > eliminating hydrogen repulsion is by far more important than the > electrostatics. > > >decompose an HB into its "electrostatic" and "covalent" contributions. An > >"ordinary" HB (OHB) has both electrostatic and covalent components, although > >it can be reasonably modelled by classical force fields by dumping everything > >into electrostatic parameters. An OHB can be neutral or charged. A charged > > -Donald Williams > > -- > Dr. Donald E. Williams email:dew01@xray5.chem.louisville.edu > Department of Chemistry > University of Louisville phone:502-852-5975 > Louisville, KY 40292 fax: 502-852-8149 > > > --- > Administrivia: This message is automatically appended by the mail exploder: > CHEMISTRY@www.ccl.net: Everybody | CHEMISTRY-REQUEST@www.ccl.net: Coordinator > MAILSERV@www.ccl.net: HELP CHEMISTRY or HELP SEARCH | Gopher: www.ccl.net 73 > Anon. ftp: www.ccl.net | CHEMISTRY-SEARCH@www.ccl.net -- archive search > Web: http://www.ccl.net/chemistry.html > --- > >