Re: CCL:Specific Buckingham parameters in Tinker

 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/