From tripos!metis!matt@uunet.UU.NET  Thu Oct  7 10:42:23 1993
From: tripos!metis!matt@uunet.UU.NET (Matt Clark)
Message-Id: <9310072142.AA06977@metis.UUCP>
To: uunet!ccl.net!chemistry@uunet.UU.NET
Subject: Re: conjugation in mol mech 
Date: Thu, 07 Oct 93 14:42:23 EDT


Neil Ostlund makes an important point about atom types in
molecular mechanics, and how different bond types may occur
between two atom types. 

The Tripos force field solves this problem by adding an extra
variable to the force field, the bond type. This applies in 
particular to bond stretch and torsion terms.

This allows more flexibility in handling conjugated systems. The 
single bond length between two sp2 carbons is about 1.47 Angstroms, 
the double bond length is 1.33 Angtroms. There are different
torsion parameters for single and aromatic bonds used in the 
case of biphenyl torsions, allowing these to be parametrized more
accurately. It isn't the same as a pi calculation, but significantly 
improves the accuracy for conjugated systems. 

I would encourage anyone developing new force fields to use this
idea; it greatly expands the possibilities for future expansion of
the force field. Organometallic modeling is much easier with 
this scheme - you can assign different bond lengths for bridging 
bonds, dative bonds etc., between the same two atom types.