Sybyl FF VdW Term

[jle "-at-" world.std.COM (Joe M Leonard)]

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 "-at-" world.std.com