[Pw_forum] Negative Phonon Dispersion after geometry optimization twice.

Nicola Marzari nicola.marzari at epfl.ch
Wed Sep 7 13:19:23 CEST 2016

On 07/09/2016 13:12, Mohammed Ghadiyali wrote:
> Thanks for the Reply.

This structure has first and foremost an instability at the M point.

The cause might just be that you are describing an incorrect structure;
or that it is a 2d material with a charge density wave instability,
or with some other instability.

If CDW the physics is not well described by GGA. Again,
read etc...

Also, you need to read about soft modes and soft phonons - e.g. the PRL
by Nardelli/Baroni mid-90s.

The general procedure requires, I think, discussing with a supervisor
or spending a few weeks understanding what phonons are, how they are
calculated, etc... without touching the code.


> The negative frequencies are not just at gamma but along the axis of k
> vector (please check attachment for phonon dispersion png)
>>> If somewhere else, you have a phase instability and a soft phonon mode
>>>(if it's a charge density wave, not evident say GGA will be good enough)
> Was not able to get what did you meant?  As for the literature almost
> all work on the 2D tin has been done using VASP.
> Can you inform me what general procedure you do recommend for the
> geometry optimization and relax.
> I am using phononpy for the phonon dispersion calculation as I was not
> able to get documentation regarding q2r.x and matdyn.x
> Regards,
> Ghadiyali Mohammed Kader.

Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project

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