[Q-e-developers] bug in phonon code?

[Giovanni Pizzi]

Dear developers,
we are experiencing a problem with the phonon code. We think that this 
may be a serious bug.
Here a short description of the problem; below, a more detailed 
description follows. Attached, all relevant inputs and outputs of the 5 
calculations of interest.

Short description:
* We have a 40-atom cell with cubic cell (and cubic space group). pw.x 
recognizes 6 symmetries (folder 01_... for the vc-relax, and 02_... for 
the restart from that vc-relax). However, then when ph.x starts (folder 
03_...), it crashes with "Error in routine set_irr_sym (1422): wrong 
representation".
* As a workaround, we slightly translate the system (folder 04_...). 
Then, pw.x just finds one symmetry (the identity), and subsequently ph.x 
starts and computes the phonons. We ask for the phonons on a 2x2x2 
q-mesh, and since no symmetries are found, all 8 qpoints are evaluated 
(folder 05_...).
* Since the system has cubic symmetry (even if pw/ph don't detect it), 
we expect that the frequencies found on equivalent q-points should be 
the same. This should be the case for the following q points (see folder 
05_...):
        N         xq(1)         xq(2)         xq(3)
        2   0.000000000   0.000000000  -0.061601673
        3   0.000000000  -0.061601673   0.000000000
        5  -0.061601673   0.000000000   0.000000000

Instead, while the frequencies of qpoints 2 and 3 are similar within 1 
cm^-1, the frequencies at qpoint 5 are quite off (there is a negative 
frequency -928.641695 [cm-1], and also a new maximum-energy frequency of 
4496.358033 [cm-1], while q points 2 and 3 have frequencies in the range 
70-740 [cm-1].

Could anyone help us in finding where the bug lies?

Best,
Andrea Cepellotti and Giovanni Pizzi
THEOS (Theory and Simulation of Materials), EPFL, Switzerland


P.S.: longer description of what we did with some more details:
the attached zip has 5 folders, starting with an incremental number 01_, 
02_, ...
* The first calculation (01_) is a vc-relax of the cell.
* The second calculation (02_) is a vc-relax, restarted from the 
coordinates of 01_ (the volume changed significantly during the vc-relax 
and we had to restart the calculation with the newly-calculated G-vectors)
Both these cells have a cubic cell (defined with ibrav=0) and a cubic 
space group: I-43m (217), checked using the 'spglib' code.
* we start a phonon calculation (03_) from the results of 02_: the 
calculation crashes as described above.
* we take the relaxed coordinates of job 02_ and translate them by the 
vector (0.01, 0.02, 0.03) angstrom. With these new coordinates, we 
perform an scf calculation (job 04_).
Note that also this system is correctly recognized by the spglib code to 
have the same cubic space group of before: I-43m (217)
* we perform a phonon calculation (job 05_) from the scf of job 04_. Due 
to queue limitations on our supercomputer, the calculation was run at 
steps of 23 hours (and stopped using the max_seconds flag) and then 
restarted multiple times. The output file of the first run is called 
'aida.out', the subsequent restarts are called 'aida-recover-N.out' with 
N ranging from 1 to 12.


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