Re: AMBER: Azobenzene: dihedral angle CCNN force field parameter

From: Adrian Roitberg <>
Date: Sat, 26 Apr 2008 17:10:25 -0400

Let's make sure we all understand what these torsional potentials mean.

They are NOT the true torsional potential that the system feels if
computed under the 'perfect force field' or a great QM hamiltonian.

They are in fact, a fudge factor introduced to correct for imperfections
in the force field.

So, there WILL be a barrier even if the torsional potential has a zero
force constant.

Now, is this barrier good enough, and are the minima in the correct
place? To answer that you must do a torsional potential scan with a good
QM method, and compare that energy profile with what the force gives you
with a 'zero' torsional potential.
The difference between those two calculations in then fit with a fourier
series. Voila, you got your tosional potential then.


Chih-Ying Lin wrote:
> Hi
> I found that the dihedral CCNN force field for the azobenzene are not quite right.
> 0 3.14 3.0 :1.CAH :1.CAM :1.NAK :1.NAL (9,11,12,13)
> With this set of parameters, the two phenl rings can rotate freely.
> Any one have idea for this?
> Thanks
> Lin
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                            Dr. Adrian E. Roitberg
                              Associate Professor
               Quantum Theory Project and Department of Chemistry
University of Florida                         PHONE 352 392-6972
P.O. Box 118435                               FAX   352 392-8722
Gainesville, FL 32611-8435                    Email
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Received on Sun Apr 27 2008 - 06:08:18 PDT
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