[gmx-developers] FW: repeatability of runs?

[Shirts, Michael R. (mrs5pt)]

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

5.04.

>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.

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