[QE-users] Wrong representation solved by nosym but implies crazy frequencies and looses phonons symetries

[Antoine Jay]

Dear all,
I ckecked the phonon frequencies at q=gamma of a 512-atoms cubic supercell of bulk silicon. (kpoint=gamma also)
As the phonon code output the "wrong representation" error, I added nosym and noinv.
The output frequencies seem to be right (80-507cm^-1) except for the last one that is 1132cm^-1
Hopefully, a simple ASR was enough to decrease this frequency down to 507cm^-1.

This calcul implies three questions:
1) How is it possible to have such a decrease between before ASR and after ASR, is the calcul wrong (input in PJ)?
2) Why is there a "wrong representation" error, whereas the atomic coordinates are exactelly symetric and not quasi-symetric?
3) Is it possible to know which modes are degenerated (nosym implies C_1 space group which looses the symetry analysis)?
For this last, I cannot use only the phonons frequencies because they are not exactelly equals, and due to the BZ folding in the supercell, several frequencies are very close from each other.

Thank you very much for your help.

Antoine Jay
LAAS-CNRS
Toulouse France
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