Subject: vdW parameters

From: Grzegorz Bakalarski <[email-masked]>
Date: Thu, 4 Jan 2001 16:30:03 +0100

Dear All,

Recently I analyzed vdW parameters in different force fields
(sybyl, few flavors from MSI, Charmm, MM3 etc) and I noticed
that the parameters differ  substantially! 
In crystallografer community the most popular are Bondi vdW radii
and they are in most cases completely different from those used in forcefields.
The only exception is sybyl forcefield, but here also parameter for
H atom is 1.5 A (instead of 1.2 A) which gives 0.6A difference for H-H
interaction.
My question is: what is origin of these (huge) differences ? Physical
origin of the vdW is known and should not change from forcefiled to 
forcefield ... 
In a case of overlapping H-H pair a repulsive force calculated using 
sybyl vdW radii is substantial, (e.g. if distance is 2.5A) but using
Bondi's radii the force  is small and even attracting 

Thanks in advance for any comments

Regards,

G.Bakalarski