Hi,
Let's make it a $0.07 worth. From a force field development point of view the
dihedral terms are usually created last, after bond, angle, L-J terms, and
partial atomic charges. As such, and in my opinion, their roles is to account
for any left over unaccounted quantum effects (e.g. anomeric effect, guache
effect, e- conjugation, etc). As Adrian said, much of the energy seen in
torsion rotation comes from the nonbonded terms, but any through-bond quantum
effects is captured by the torsion terms. I would be careful to say that
torsion terms are just a fudge factor, because they do model some chemistry
in the same way that a bond term models stretching. Never-the-less, they also
are used to to correct the rotational PE curve for the physics that has not
been accounted for in the bond, angle, and nonbonded terms. They are also
often adjusted in order to obtain better agreement to an experimental
observable in an MD simulation (eg reproducing secondary elements in
proteins), making their use as a fudge factor even more.
The interpretation of a V1, V2, or V3 term is useful and is based on the
orbital hybridization model (eg the rotation about the central bond in a
sp3-sp2=sp2-sp3 torsion is represented by a V2 term). The differences in how
one views each force field term, and how one approaches their
parameterization is the reason why there is an art to parameterization :) .
Cheers,
Karl
------------------------------------
Karl N. Kirschner, Ph.D.
Fraunhofer-Institute for Algorithms
and Scientific Computing - SCAI
Department of Simulation Engineering
Schloss Birlinghoven
53754 Sankt Augustin, Germany
Tel: +49 (0) 2241-14-2052
Fax: +49 (0) 2241-14-1328
------------------------------------
On Sunday 07 June 2009 18:14, Carlos Simmerling wrote:
> I add my 2 cents in support of Adrian's 3- clearly the DIHED energy is not
> the energy for rotating around a bond. I also see this misinterpreted in
> the literature quite often. As Adrian says, the DIHED is an empirical
> correction term in most cases.
>
> On Sun, Jun 7, 2009 at 12:09 PM, Adrian Roitberg
<roitberg.qtp.ufl.edu>wrote:
> > 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.
> >
> > Adrian
> >
> > 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
> >>
> >> _______________________________________________
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> >> http://lists.ambermd.org/mailman/listinfo/amber
> >
> > --
> > 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 adrian.qtp.ufl.edu
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Received on Tue Jun 09 2009 - 01:07:13 PDT
