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 biaxial strain also have no negative frequency in phonon dispersion but 1% tensile biaxial strain phonon dispersion shows imaginary modes in phonon. How is it possible that 2% and 0% tensile biaxial phonon dispersion are stable but 1% biaxial phonon dispersion would be unstable ?
Thanks
Sourav Rudra
PhD student
JNCASR, Bengaluru
INDIA
problem in phonon calculation
Moderator: hlee

 Posts: 14
 Joined: Mon Jul 11, 2022 8:48 am
 Affiliation: PhD student
Re: problem in phonon calculation
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.quantumespresso.stage.24h.it/usersforum/.
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
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.quantumespresso.stage.24h.it/usersforum/.
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

 Posts: 14
 Joined: Mon Jul 11, 2022 8:48 am
 Affiliation: PhD student
Re: problem in phonon calculation
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 1.0d14 for both scf and phonon. with using same parameter in 2% and 1% biaxial tensile case but 2% is stable dynamically but 1% is unstable.
Thanks
Sourav
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 1.0d14 for both scf and phonon. with using same parameter in 2% and 1% biaxial tensile case but 2% is stable dynamically but 1% is unstable.
Thanks
Sourav
 Attachments

 phonondispwesion2%.PNG (17.34 KiB) Viewed 350 times

 phonondispwesion1%.PNG (15.83 KiB) Viewed 350 times

 Posts: 14
 Joined: Mon Jul 11, 2022 8:48 am
 Affiliation: PhD student
Re: problem in phonon calculation
The path in which phonon dispersion is calculated is X to Gamma to Z, so origin to Z direction is problematic in this case.