[ERROR] negative phonon frequency in TDEP

[WenjiangZhou]

Hello, Dear TDEP developer,
I used TDEP to extract the phonon band, but I meet a negative phonon frequency. I'm sure my material is stable and the phonon from DFPT (VASP) is quite good. Do you have any suggestions to help me solve this question? Thank you very much.

TDEP:

DFPT:

[mjv500]

Hello, you are running a 2D material, and it seems you have issues with the interpolation. For TDEP there is a flag you can play with ( no rotational), for 2D materials but also hermiticity and Huang invariances. Try turning them all off to begin with. https://tdep-developers.github.io/tdep/program/extract_forceconstants/ Matthieu

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On Mon, Jul 29, 2024 at 2:24 PM WenjiangZhou ***@***.***> wrote: Hello, Dear TDEP developer, I used TDEP to extract the phonon band, but I meet a negative phonon frequency. I'm sure my material is stable and the phonon from DFPT (VASP) is quite good. Do you have any suggestions to help me solve this question? Thank you very much. TDEP: image.png (view on web) <https://github.com/user-attachments/assets/5a127015-fb5e-46f0-864c-7937130e1ea3> DFPT: image.png (view on web) <https://github.com/user-attachments/assets/e4d548b9-7715-45ee-82ad-4fce9978c5af> — Reply to this email directly, view it on GitHub <#81>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABWOT3YSKJXFDOZ3N5F6HYTZOYYBFAVCNFSM6AAAAABLUFLRL6VHI2DSMVQWIX3LMV43ASLTON2WKOZSGQZTKMRXHAYTQOA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Professor Matthieu J Verstraete Fellow, American Physical Society Chair, Steering Committee, European Theoretical Spectroscopy Facility www.etsf.eu Alumnus Fellow, Young Academy of Europe yacadeuro.org Institute for Theoretical Physics (ITP) Department of Physics Buys Ballot Gebouw/Building, Princetonplein 5, office University of Utrecht, 3584 CC Utrecht ITP Secretariat: +31 30 253 5928 E-mail: ***@***.*** Group web page (Liège): http://www.nanomat.ulg.ac.be/ Nanomat lab, Q-Mat center, Université de Liège Département de Physique, Bat. B5a, 4/50 Allée du 6 août, 19 B-4000 Sart Tilman, Liège Belgium Phone : +32 4 366 90 17 European Theoretical Spectroscopy Facility (ETSF) Mail : ***@***.*** ***@***.*** ***@***.***

[WenjiangZhou]

Thank you very much. I've solved this issue when I turn off the Huang invariances. I also find that you think deriving the proper invariances for a two-dimensional material is necessary. I'd like to contribute more if have chances.