[QE-users] Negative frequencies on a supercell

[Fabio Costa]

Dear all

I'm trying to calculate the phonon modes for a doped system within the supercell approach. The structure is composed by eight atoms, with one being replaced by the doping specimen.

I started the calculation by relaxing the structure, and then I perform the phonon calculation, followed by q2r.x and matdyn.x steps.

It is well known that negative frequencies in the phonon spectra usually means that the system in case is unstable. But in this particular case, there are three optic modes that are completely negative. This is a very strange behavior, which I've never seen before.

It is possible that this only means that the structure is not stable. But as I'm not very experienced with calculations with supercells, I can't surely say wether this is an instability or the result of some mistake on the settings.

The relevant files are stored here: https://github.com/fabiocosta4/soft-modes.git

Hopefully someone will be able to shed some light on this matter. In advance, many thanks for your time and assistance.

Regards,
Fábio Costa
Physics Institute, Federal University of Bahia

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