Subject: Re: CCL:Dreiding Models

From: Alexandre Hocquet <[email-masked]>
Date: Tue, 8 Apr 1997 22:15:11 +0400

I guess we are talking about two different things, here.

- DREIDING (capital letters ?) is an empirical force field, ie a set of
equations and parameters
 designed to perform molecular mechanics calculations (Mayo et al.
J.Phys.Chem., 1990, 94,
 8897-8909). Its ambition is to provide reasonable results with a minimum of
general
parameters, within the entire periodic table.

- Dreiding models are physical metallic molecular models. Compared to
plastic models
 (like Cochranes) they provide no choice for L0 in the stretching term and
the bending 
Ktheta are nearly infinite (the only way you can twist them is by a
torsional term). 
On the other hand, the Vn terms for torsion are very low (rotation around
bonds are free)
while you can count on friction to fix the molecule into a conformation in
Cochranes.

For example, Dreiding models are very useful for the 1,4-cyclohexadiene
trick, because they
 spectacularly fail to find a planar minimum, whereas Cochranes can be
forced on a planar
structure simply because plastic atoms allow a light bend deformation and
metallic ones do not.

Does the DREIDING force field corrrectly predict a boatlike conformation ? I
suppose so.
Alexandre Hocquet
Laboratorio de Cristalografia
Facultad de Ciencias Fisicas
Universidad de Chile
Blanco Encalada, 2008
Santiago Centro
Chile
fono : 56 2 678 45 19
fax : 56 2 696 73 59
email : [email-masked]