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

[Merlin Meheut]

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.

> 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