[gmx-users] Identical energies generated in a rerun calculation ... but ...

[Berk Hess]

Hi,

I can fix, but I am currently very busy, so it might take some time.

Berk

> Date: Fri, 24 Apr 2009 17:56:56 +1000
> From: Mark.Abraham at anu.edu.au
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] Identical energies generated in a rerun calculation	...	but ...
> 
> Mark Abraham wrote:
> > 
> > OK I have some confirmation of a possible bug here. Using 4.0.4 to do 
> > reruns on the same positions-only NPT peptide+water trajectory with the 
> > same run input file:
> > 
> > a) compiled without MPI, a single-processor rerun worked correctly, 
> > including "zero" KE and temperature at each frame
> > 
> > b) compiled with MPI, a single-processor run worked correctly, including 
> > zero KE and temperature, and agreed with a) within machine precision
> > 
> > c) compiled with MPI, a 4-processor run worked incorrectly : an 
> > approximately-correct temperature and plausible positive KE were 
> > reported, all PE terms were identical to about machine precision with 
> > the first step of a) and b), and the reported pressure was different.
> > 
> > Thus it seems that a multi-processor mdrun is not updating the structure 
> > for subsequent steps in the loop over structures, and/or is getting some 
> >  KE from somewhere that a single-processor calculation is not.
> > 
> > I'll step through c) with a debugger tomorrow.
> 
> d) compiled with MPI, a 4-processor run using particle decomposition 
> worked correctly, agreeing with a).
> 
> Further, c) has the *same* plausible positive KE at each step.
> 
>  From stepping through a run, I think the rerun DD problem arises in 
> that a rerun loads the data from the rerun trajectory into rerun_fr, and 
> later copies those into state, and not into state_global. state_global 
> is initialized to that of the .tpr file (which *has* velocities), which 
> is used for the DD initialization, and state_global is never 
> subsequently updated. So, for each rerun step, the same .tpr state gets 
> propagated, which leads to all the symptoms I describe above. The KE 
> comes from the velocities in the .tpr file, and is thus constant.
> 
> So, a preliminary work-around is to use mdrun -rerun -pd to get particle 
> decomposition.
> 
> I tried to hack a fix for the DD code. It seemed that using
> 
> for (i=0; i<state_global->natoms; i++)
>    copy_rvec(rerun_fr.x[i],state_global.x[i])
> 
> before about line 1060 of do_md() in src/kernel/md.c should do the 
> trick, since with bMasterState set for a rerun, dd_partition_system() 
> should propagate state_global to the right places. However I got a 
> segfault in that copy_rvec with i==0, despite state_global.x being 
> allocated and of the right dimensions according to Totalview's memory 
> debugger.
> 
> I'll file a bugzilla in any case.
> 
> Mark
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