Are they negative at gamma, close to gamma, or somewhere.
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). Read the
literature.
If at gamma, you haven't used one of the sum rules that forces it to be
zero.
If very close to gamma, it is a known problem, due to the high
sensitivity to
numerical noise for quadratic (typically) phonons that would have almost
zero energy.
nicola
On 07/09/2016 10:47, Mohammed Ghadiyali wrote:
> Dear All,
>
> After performing the geometry optimization twice, I am still getting
> negative phonon dispersion.
>
>
> For first geometry optimization I used following tags
>
> calculation = 'vc-relax'
>
> ion_dynamics = 'bfgs'
>
> cell_dofree = 'shape'
>
>
> And for second time I used following tags
>
> calculation = vc-relax'
>
> ion_dynamics = 'bfgs'
>
> cell_dofree = '2Dxy'
>
>
> The system I am working on is a 2D buckled tin, can someone help me out
> and inform me as how should one proceed with
>
> geometry optimization. For Phonon Dispersion calculation I have used
> phonon package induced in QE as well as phononpy.
>
> I have also included my input file, please check the attachment for the
> same.
>
>
>
> Regards,
>
> Ghadiyali Mohammed Kader.
>
>
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>
--
----------------------------------------------------------------------
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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