[gmx-developers] Double precision restarts are exact, single precision are not in gromacs 3.2.1

Berk Hess hessb at mpip-mainz.mpg.de
Mon Jul 25 10:40:43 CEST 2005

Mark Abraham wrote:

> Hi,
> After posting twice to gmx-users about this problem and apparently 
> being ignored, I've done yet more testing and discovered a clear 
> difference in the way single precision and double precision gromacs 
> handle MD restarts in version 3.2.1 (using Berendsen thermostat and 
> barostat, and PME, on Pentium 4 architecture).
> Single precision restarts using the previous run's trajectory (.trr) 
> and energy (.edr) files to obtain the configuration and other 
> information (and using unconstrained_start = yes, and gen_vel = no in 
> the .mdp file to make sure that the old configuration is preserved) do 
> in fact reproduce the configuration exactly, in that the energy 
> components calculated by the force field are identical in Step 0 of 
> the restart with those of the final Step in the previous run. However 
> the total pressure is different, and by 10 steps later (my next output 
> interval), all energy components as well as the pressure have diverged 
> from those obtained for a run. This suggests that values used to 
> calculate the total pressure (e.g. the virial) are not being preserved 
> correctly across the restart, and this is subsequently perturbing the 
> system, at least through the barostat.
> However double precision gromacs performs "correctly" - all energy 
> components and the total pressure are reproduced correctly in Step 0 
> of the restart, and future steps agree exactly (within output 
> precision over only a few tens of MD steps) with those obtained from a 
> run that extended over the restart time from the original starting point.
> This suggests that something is being written or read at an 
> inappropriate precision in the .edr files. Can this be brought to the 
> attention of somebody who knows how that code is supposed to work, 
> please? They may wish to see if they can reproduce my observations & 
> work out what is wrong.

I have always gotten exact restarts in single precision.
Could you try restarting with a tpr obtained from tpbconv with a trr and 
edr file?

I don't think there is a difference between single and double precision.
You might not see the difference in double precision as it will be in 
the 10 decimal
or so which is often not printed.


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