[gmx-users] force evaluation

Florian Dommert dommert at fias.uni-frankfurt.de
Tue Jan 6 09:59:57 CET 2009

```Hello,

since I am also need direct access to the forces and play around a
little bit I realized that this is not that easy. However if you
set "nstfout=1", you should get the forces in every step.

But I have another question, regarding the direct part of the coulomb forces.
Since we have worked out an error estimation for the SPME algorithm,
that allows us to estimate the absolute error introduced by the
interpolation and the cutoff. We wrote a little tool to test the
estimate and it works very well. But comparing the forces calculated by
this tool and Gromacs yields a root mean square difference of the
forces of 10^-4. That is a very big difference, because the precision
of the forces should be 10^-12. Now the question arises, which forces
are "wrong" ? Since I talk about the forces of the first step before
any integration, only the force calculation can introduce the
differences and I can not imagine, that the compiler optimizations are
responsible. Moreover the comparison was done, using the Ewald sum, so
there is no FFT involved, that can cause the differences.
The next point I thought about were the tabulated potentials, but
turning this of in the source code yielded even a larger difference.

The problem is that the error estimate of Kolafa and Perram (
http://dx.doi.org/10.1080/08927029208049126 ) for the
Ewald sum underestimates the error introduced by Gromacs, while our
tool fits to the estimate.

Can anybody imagine the source of difference ?
Furthermore I have found no possiblity to turn of the tabulation of the
coulombic forces in the mdp options, did I miss it or is there no
possibility ?
Finally it would be very interested in a possiblity to switch off the
vdw interactions easily. So far I used an appropiate topology files with C6 and
C12 set to zero. Is there an easier solution as using a table filled
with zeros ?

Cheers,

Flo

* Mark Abraham <Mark.Abraham at anu.edu.au> [2009-01-06 08:45:05 +1100]:

> Silvia Crivelli wrote:
>> Hello,
>> I need to calculate the forces (derivatives) for every atom in a protein
>> as this is being locally minimized using mdrun.
>> When I run mdrun I can get the maximum force F-max and the atom on which
>> this
>> force is evaluated printed in the log file (for example,
>> "F-max             =  2.20323e+03 on atom 2641 ".)
>> I need to be able to compute the force value on every atom of the protein.
>> How can I do this with Gromacs?
>> I set the parameter nstfout = 10 in my .mdp file but this doesn't seem to
>> make a difference.

>
> "nstfout = 10" writes the per-atom forces to the trajectory file every 10
> steps, which you can verify with gmxcheck and/or gmxdump. I haven't verified
> that these actually get written for energy minimization, and they probably
> wouldn't get written at all if your EM doesn't get to 10 steps....
>
> Mark
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--
Florian Dommert
Dipl.-Phys.

Computational and Theoretical Softmatter & Biophysics group

Johann-Wolfgang-Goethe University

Ruth-Moufang-Str. 1
60438 Frankfurt am Main

Phone: +49(0)69 / 798 - 47529
Fax:   +49(0)69 / 798 - 47611

EMail: dommert at fias.uni-frankfurt.de
Home: http://fias.uni-frankfurt.de/~simbio/Florian_Dommert
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