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Date: Mon, 31 Jul 2000 11:50:57 +0200 (MDT)
From: Harald Svedung <svedung -A_T- phc.chalmers.se>
To: Didier MATHIEU <mathieu _-at-_)ripault.cea.fr>
cc: chemistry#* at *#ccl.net
Subject: Re: CCL:Specific Buckingham parameters in Tinker
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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 # - at - # svedung.pp.se
SE-412 96 Goeteborg, Sweden             www.che.chalmers.se/~svedung/