From chemistry-request@ccl.net Thu Jun 25 15:24:14 1992
Date: Thu, 25 Jun 92 14:29:28 -0400
From: jle@world.std.COM (Joe M Leonard)
Subject: Sybyl FF VdW Term
To: chemistry@ccl.net
Status: RO

I've been looking over the J Comp Chem article regarding the
Sybyl 5.2 force field, and I'm curious as to the functional form
of the VdW term.  The paper lists:

        E = k(ij) * (1/a**12 - 2/a**6)

where

        a = r(ij) / (R(i) + R(j))

with R being the defined VdW radius.  This term raises the energy (is
repulsive) when r(ij) < (R(i) + R(j)), but lowers the energy when
the reverse is true.  Is this the correct behavior, in that the force
field is parameterized assuming such longer-distance E-lowering?

This seems odd, as increasing the number of atoms in a structure
serves to lower the E?  Would it not be more reasonable to have the
VdW term drop to zero rather than such negative numbers?

I'm not at all familiar with the Sybyl Force Field, and would appreciate
any comments from those who are - or from Tripos directly...

Thanks in advance,

Joe Leonard
jle@world.std.com