problem in phonon calculation

[souravrudra]

Hello developers,
I am trying to calculate the phonon dispersion of strained system. The unstained system is stable with no negative frequency in the phonon dispersion. 2% tensile bi-axial strain also have no negative frequency in phonon dispersion but 1% tensile bi-axial strain phonon dispersion shows imaginary modes in phonon. How is it possible that 2% and 0% tensile bi-axial phonon dispersion are stable but 1% bi-axial phonon dispersion would be unstable ?

Thanks
Sourav Rudra
PhD student
JNCASR, Bengaluru
INDIA

[hpaudya1]

Hi Sourav,

I guess you are talking about the phonon calculation using quantum espresso (not the phonon interpolation using EPW), right? If yes, you may get better suggestions from quantum espresso forum; https://www.quantum-espresso.stage.24h.it/users-forum/.

At this point, I recommend you sharing your phonon dispersion plots so that exports can clearly see what you are talking about. Usually this type of issue appears because of convergence, so in the mean time I also recommend you to perform some convergence tests of your phonon calculations.

Best,
Hari

[souravrudra]

Hello,
I have attached here the phonon dispersion generated from Quantum Espresso, Yes I am saying about Phonon dispersion from Quantum Espresso.
I have used 24x24x24 kmesh for scf and 6x6x6 qmesh for phonon. Conv_thr is 1.0d-14 for both scf and phonon. with using same parameter in 2% and 1% bi-axial tensile case but 2% is stable dynamically but 1% is unstable.

Thanks
Sourav

[souravrudra]

The path in which phonon dispersion is calculated is X to Gamma to Z, so origin to Z direction is problematic in this case.