[Pw_forum] Negative Phonon Frequencies - Numerical instability or unstable structure

[ulrich.aschauer at dcb.unibe.ch]

Dear Mohammed,

Have you computed your force constants for a supercell or are you inter/extrapolating q≠Gamma from the --dim=“1 1 1” force constants? If it’s 1 1 1, try to make a supercell that will give you access to the zone-boundary points so that your interpolation becomes better.

Best regards,
-uli
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SNF Prof. Dr. Ulrich Aschauer
Universität Bern
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ulrich.aschauer at dcb.unibe.ch<mailto:ulrich.aschauer at dcb.unibe.ch>
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On 5 Dec 2016, at 09:25, Pietro Delugas <pdelugas at sissa.it<mailto:pdelugas at sissa.it>> wrote:

Hello Mohammed

If the phonon were computed with phononpy for any numerical problem maybe you will find more help in the forum of phononpy users.

Could you provide more information about the structure ?  Is it 3D or 2D ?


greetings - pietro

On 05/12/2016 08:23, Mohammed Ghadiyali wrote:
Dear All,

I am working on a bilayer tin system and has performed geometry relaxation using Quantum Espresso 5.4.0 of the structure via:


1. One relax calculation - for coordinate relaxation

2. One vc-relax calculation with cell_do_free as 2Dxy - for coordinate and cell shape relaxation

3. Two vc-relax calculation with cell_do_free as shape - for coordinate and cell shape relaxation with constant volume.


these calculations were performed with, ecutwfc = 60 Ry and K-point mesh as 10 10 1, 1 1 1.

Further, phonon calculations were performed using the phononpy 1.11.0 package with the same parameters. The

structure seem to gotten relaxed but it still has some negative frequencies near the Gamma point, are these's negative

frequencies numerical instabilities or the structure is unstable.


I am attaching the phonon dispersion file. Please advice.


Regards,

Ghadiyali Mohammed Kader.

Research Scholar,

University of Mumbai.




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