[Pw_forum] How to parallelize phonons

["Alejandro Rébola"]

Dear all,

I'm trying to run phonon calculations using ph.w on a cluster (NERSC), and
since I have a big number of atoms I wanted to parallelize it as much and
efficiently as possible. I've been reading the documentation, and I've
seen the GRID example, I was going to use this but then I saw the
following at the end of the INPUT_PH.html file:

On parallel machines the q point and the irreps calculations can be split
automatically. The procedure is the following:

" [...]

1) run pw.x with nproc processors and npools pools.
2) run ph.x with nproc*nimage processors, npools pools and nimage images.
3) run ph.x with the same input and recover=.true. on nproc processors
   and npools pools and only one image.

During the first ph.x run the phonon code split the total amount of
work into nimage copies. Each image runs with different q and/or
representations.  The second run of ph.x is the final run that
collects all the data calculated by the images and writes the files
with the dynamical matrices."

Since I'm not very familiar with the terminology used here (I'm completely
lost) I would like to ask some questions:
1) If I'm running on a cluster like NERSC, would I get any advantage from
using the GRID method or should I just use the method outlined above?
2) In that case, what does it mean by images for a phonon calculation (I'm
just familiar with images for NEB). Where could I find more documentation
about this or some example?
Thank you in advance,

Alejandro Rébola