[gmx-users] simulations backward in time

Jussi Lehtola jussi.lehtola at helsinki.fi
Thu Aug 21 17:03:49 CEST 2008

On Thu, 2008-08-21 at 10:32 -0400, Sumanth Jamadagni wrote:
> Using a negative 'dt' in the mdp file does not not seem to cause any
> simulation problems (tested on a waterbox- doesn't crash). I am not sure
> if the system is evolving backwards in time though. I am not sure how to
> check that.  

So is the purpose of this to run an NVE simulation? Since if you have
any thermostats enabled the sign of the time step doesn't matter since
the simulation is "screwed up" anyhow..

Also when using thermostats inverting the sign of the time step may
cause wrong behaviour since then the difference to the desired value may
increase if the time step is assumed positive in the algorithm.

Physically, inverting the initial velocity corresponds to inverting the
direction of time. Of course you will not get back the same
configurations as before due to rounding errors and a chaotic system.
Jussi Lehtola, FM, Tohtorikoulutettava
Fysiikan laitos, Helsingin Yliopisto
jussi.lehtola at helsinki.fi, p. 191 50673
Mr. Jussi Lehtola, M. Sc., Doctoral Student
Department of Physics, University of Helsinki, Finland
jussi.lehtola at helsinki.fi

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