[QE-users] scf convergency depends on the number of mpi processes !!!

[Paolo Giannozzi]

In systems with a difficult self-consistency, it is possible that the 
small numerical differences, coming from execution on different numbers 
of processors, are sufficient to drive the system towards 
non-convergence, or convergence towards a different final state (DFT+U 
seems to be especially unstable in this respect).

If you are using some exotic, little tested feature, it is conceivable 
that some overlooked bug in parallelization exists. In this case, the 
problem is easily reproducible and clearly bound to a specific feature, 
though.

I do not see any other possibility

Paolo

On 28/11/2022 17:05, Mahmoud Payami Shabestari via users wrote:
> Hi All,
> I am experiencing that the success in scf cycle depends on the number of 
> processes used in mpirun. That is, for example, for a given scf input 
> when I use "mpirun -np *2* pw.x < input.in",
> it gives the converged result in a reasonable number of iteration; but, 
> on the other hand, if I instead use "mpirun -np *8* pw.x < input.in", 
> the system does not converge numerically.
> First I thought it is somehow a problem of numerical error accumulation. 
> So, I used in compilation of QE the FFTW3 as prescribed in the manual. 
> And even used "-lfftw3l" (long-double) to cure this problem, but some 
> other numerical problems happened in vc-relax job.
> I would appreciate if anybody helps to understand the roots of this 
> dependency and how to control it.
> Bests,
> Mahmoud Payami
> NSTRI, AEOI, Tehran, Iran
> Email: mpayami at aeoi.org.ir
> Phone: +98 (0)21 82066504
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216