[Pw_forum] Using -nimage with phonon at q=0

[Andrea Dal Corso]

On Thu, 2015-08-06 at 14:00 +0200, Merlin Meheut wrote:
> On 05/08/2015 18:50, Andrea Dal Corso wrote:
> > On Wed, 2015-08-05 at 17:18 +0200, Merlin Meheut wrote:
> >> Dear PWSCF users,
> >>
> >> I recently discovered with great interest the possibilities to
> >> parallelize phonon calculations using the -nimage option of ph.x.
> >> (example given in espresso-4.3.2/examples/GRID_examples).
> >>
> > GRID examples refer to grid splitting, meaning that you split the
> > calculation using start_irr, last_irr etc. and in principle the
> > calculation can run in different machines. Then you have to collect the
> > files  yourself to finally obtain the results.
> 
> Sorry I mixed up both. Actually, I used -nimage (as explained in 
> Doc/INPUT_PH.txt of version 4.3.2), but I would like to restart with 
> grid splitting.
> Thank you for the reference to the example.
> 
> >> However, I had a problem when performing calculations at gamma-point:
> >> for other q-points (therefore with epsil=.false. and zue=.false.)
> >> everything went as planned, but with q=0 (and epsil=.true. and
> >> zue=.true.), this just did not work. I took 80 processors divided into 4
> >> images, and instead of dividing the different representations into 4
> >> pools, the four groups of processors realized the same calculation,
> >> computing the same representations. I killed the calculation at some
> >> point (I have computed the electric fields, effective charges and 218
> >> representations out of 564). I would like now to finish the computation
> >> without redoing it, and I have several questions to achieve this goal:
> >>
> > Are you sure that all the images made the same phonon calculations, or
> > all images made the electric field calculation but the phonon modes were
> > different?
> 
> The names of the partial dynamical matrices (dynmat.1.$irr.xml) are the 
> same, the files (e.g. _ph0/LiClMag2-1.phsave/dynmat.1.100.xml and
> _ph1/LiClMag2-1.phsave/dynmat.1.100.xml) are almost identical, and the 
> calculation lasted more than it should have (each of the 4 separate 
> images has computed more than 141 representations, which is one quarter 
> of the total). So as far as I understand it, all the images made 
> electric field calculation and started making the same phonon mode 
> calculations in parallel.
> 

Thank you for reporting this. What is not supported is the option
-nimage together with gamma-gamma tricks that you are using in the gamma
calculation.
One easy solution is to set nogg=.TRUE. in the input of the phonon if
you do not need the gamma-gamma tricks to speed-up the calculation.


Andrea




> > In this case, since you stopped the calculation, you can only
> > collect the .xml files in a single _ph0 directory and restart without
> > images. The restart with images is still poorly supported.
> 
>   I would like to restart without -nimage (since it seems to fail in 
> this particular case), but with grid splitting. Do you think this is 
> presomptuous?
> By the way, I had a positive experience restarting with -nimage on 
> another run (at q neq 0) .
> 
> > In the last version of QE, the dielectric constant and effective charges
> > are saved in the tensors.xml file in the _ph0 directory.
> 
> Thank you very much for your help!
> 
> Best regards,
> 
> 
> -- 
> Merlin Méheut, Géosciences et Environnement Toulouse,
> OMP, 14 avenue Edouard Belin, 31400 Toulouse, France
> 
> phone +33 (0)5 61 33 26 17, fax +33 (0)5 61 33 25 60
> 
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-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Bonomea 265              Fax. 0039-040-3787249
I-34136 Trieste (Italy)             e-mail: dalcorso at sissa.it