[gmx-developers] FW: repeatability of runs?

Shirts, Michael R. (mrs5pt) mrs5pt at eservices.virginia.edu
Mon Feb 16 19:29:50 CET 2015

>What version(s) are we talking about here? A released 5.0 or git master


>The -reprod option tries to remove any source of divergence, such as
>FFTW auto-tuning and dynamic load balancing. Any single bit change will
>cause differences that diverge exponentially due to the chaotic nature
>of MD. But you should not see any "strange" results, such as
>pressures/virials that are higher than expected.

-reprod still gives diverging results, as soon as the 10th step (first
time energy is summed).  The differences at that point are very small -
only appearing in the virials.  So I highly doubt there are any problems
with the physics -- it's purely a mathematical result of a bit getting
rounded one way or they other, and then chaos taking it's toll.

I'll file an issue at (low) priority so that other people can take a look.
Not having the ability to reproduce (even on the same machine) does make
it hard to debug things that only show up after multiple NS simulations.

Michael Shirts
Associate Professor
Department of Chemical Engineering
University of Virginia
michael.shirts at virginia.edu
(434) 243-1821

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