Warning "break level degeneracy" in BSE calculation

[nick_damian00]

Dear Yambo Community Forum,

I am reaching you out because in the past months I have been using yambo version 5.3.0 to perform BSE calculations for MoSe2 monolayer and I encountered a warning I am not able to explain.

With the code version 5.3.0 I noticed that, when using the band range [45;50] in BSEBands (i.e. 2 bands in valence and 4 bands in conduction, since the system I am studying has the top valence in correspondence of the band index 46), I encounter the following warning:

Code: Select all

 <04s> P1: [05] Coloumb potential CutOff slab Z
 <04s> P1: [WARNING] [BS] User bands 45-50 break level degeneracy

This occurs both at Gamma and at finite q.

However, if I perform the calculation with the band range [45;48], such a warning does not occur.
Comparing the BSE eigenvalues obtained with yambopy, I see the same qualitative behaviour (and in particular the degeneracies) with both band ranges.

Should I worry about this warning?
What is its cause?

At the following GoogleDrive link:
https://drive.google.com/drive/folders/ ... drive_link
you can find the BSE input file, the BSE eigenvalues from yambopy with the two band ranges, one of the logs and the report file for the calculation at Gamma with the version 5.3.0.
Let me know if any further information is needed.

Thank you for your attention.
Best regards,

Niccolò Damian

[Davide Sangalli]

This is just a general warning, that selecting band range [45:50] you are including state 50 but not state 51, which is likely degenerate in energy with state 50.

Doing this might cause issues, in particular if you are studying the behavior of BSE eigenstates where the band 50 is involved.

If you checked that including or not band 50 (and 49) does not affect the results, you can safely ignore such warning

[nick_damian00]

Dear Doctor Davide Sangalli,
thank you very much for your fast and clear reply!

Best regards,

Niccolò Damian