From chemistry-request@server.ccl.net Mon Jul 31 05:51:18 2000 Received: from angelos.phc.chalmers.se (angelos.phc.chalmers.se [129.16.97.41]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id FAA16105 for ; Mon, 31 Jul 2000 05:51:17 -0400 Received: from fkrs2.phc.chalmers.se (fkrs2.phc.chalmers.se [129.16.97.48]) by angelos.phc.chalmers.se (8.9.3/8.9.3) with ESMTP id LAA14323; Mon, 31 Jul 2000 11:51:01 +0200 Received: (from svedung@localhost) by fkrs2.phc.chalmers.se (8.8.8/8.8.8) id LAA17071; Mon, 31 Jul 2000 11:50:59 +0200 Date: Mon, 31 Jul 2000 11:50:57 +0200 (MDT) From: Harald Svedung To: Didier MATHIEU cc: chemistry@ccl.net Subject: Re: CCL:Specific Buckingham parameters in Tinker In-Reply-To: <3969845C.FD83E36@ripault.cea.fr> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Hi, To apply the "usual combination rules", the Buckingham parameters can be converted to "usual" equilibrium distance, rm, and well depth, eps, (plus the additional hardness parameter, B ): A = 6 eps Exp(B) / (B-6) C = B eps rm^6 / (B-6) The B parameter typically varies between 10 and 15 depending on the system and it might be that some combination rule could be applied. >From preliminary calcualtions it's seen that the hetrogenous values for noble gas combinations falls between the corresponding homogenous vales as expected. An overlap integral treatment might give a suitable candidate combination rule. To avoid the "inner attractive trap" in the Buckingham potential, it can be modified as E=Aexp(-BR/rm)-C/((R/rm)^2+(0.25)^2)^3 I've found the value 0.25 to just cancel any tendency to form a local minima in the repulsive wall for B values down to 10. For what it's worth ;-) so long /Harald On Mon, 10 Jul 2000, Didier MATHIEU wrote: > Hello, > > As a first step in the modelling of a family of organic crystals, I > consider using the simple Buckingham potential of Gavezzotti and > Filippini (in: Computational Approaches in Supramolecular Chemistry, > G. Wipff, ed. Kluwer, pp. 51-62 (1994)). > However, the A,B,C parameters in the expression E=Aexp(-BR)-C/R^6 do not > obey the usual combinations rules. > On the other hand, it seems that the Tinker program I use does expect > such rules. > Is it nonetheless possible to specify independent A,B,C parameters for > each atom pair to the program, or is it necessary to modify the programs > in order to introduce the extra degree of freedom required ? > > Sincerely yours. > > -- > Didier MATHIEU > CEA - Le Ripault, BP 16 > 37260 Monts (France) > Tel. 33(0)2.47.34.41.85 > > > Harald Svedung (Ph.Lic.) phone: +46-31-7722816 Department of Chemistry fax: +46-31-167194 Physical Chemistry home phone: +46-31-240897, +46-709223206 Goeteborg University home e-mail: harald.svedung@svedung.pp.se SE-412 96 Goeteborg, Sweden www.che.chalmers.se/~svedung/