AMBER Archive (2009)

Subject: Re: [AMBER] average torsional energy

From: Adrian Roitberg (
Date: Sun Jun 07 2009 - 11:09:14 CDT

Francesco and all.
I am not fully sure I understand your question, but I will give a more
general answer.

My very strong opinion, shared by most practitioners, is that the DIHED
term (torsions) can never be interpreted in ANY way.
I also personally believe that one should be very careful in
interpreting the other terms also (vdw, elec,etc) but that is a more
complex discussion.
Why is DIHED particularly bad? because it is a clear fudge term. While
one can assign a physical interpretation to the other terms (after all,
we have the 'idea' of a bond in our heads), the torsional potential
could be thought as if if comes from the electrostatics and vdw terms
only. The torsional term itself is simply a correction to get the force
field to match a QM torsional potential.

In short, the torsional term is NOT the torsional potential ! The
torsional potential, from where minima and barriers are extracted, comes
from the total energy vs torsional angle: a scan.

This usual confusion has some troublesome uses in the literature. Some
people will set the torsional term to ZERO, and call the new potential
one with 'reduced barriers'. This is non-sense. Setting a torsional term
to ZERO could do a number of things including: changing the location of
the minima, changing the number of minima, increasing the barriers,
decreasing the barriers, etc etc.

Just my three cents' worth of opninion.


Francesco Pietra wrote:
> My I ask about the practice of correctly evaluating from the DIHED
> term (with Amber 9 or 10) the average torsional energy for an alpha
> helical stretch where a major bending at a GLY residue is observed?
> thanks
> francesco pietra
> _______________________________________________
> AMBER mailing list

                            Dr. Adrian E. Roitberg
                              Associate Professor
                             Quantum Theory Project
                            Department of Chemistry

Senior Editor. Journal of Physical Chemistry American Chemical Society

University of Florida PHONE 352 392-6972 P.O. Box 118435 FAX 352 392-8722 Gainesville, FL 32611-8435 Email

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