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

[Mohammed Ghadiyali]

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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