[gpaw-users] gw_skn and eshift in BSE

[Anders Christian Riis-Jensen]

Dear Jingjing,

If you define eshift it is not necessary to define gw_skn.
eshift will simply apply a constant scissors operators to all transition energies with the value you define.

For small band gap systems we have also before encountered "negative frequencies" within the BSE. This happens if the calculated exciton binding energy for the lowest (or one of the lowest) energy transition is larger than the electronic band gap. The physical reasoning is not straightforward, however such a result suggests that the  considered system is not the lowest energy ("ground state") system. If you are further interested I refer to "excitonic insulators" in the literature for more information on this phase.

best regards
Anders C. Riis-Jensen
________________________________
Fra: Jens J?rgen Mortensen <jensj at fysik.dtu.dk>
Sendt: 19. juni 2020 14:32
Til: Anders Christian Riis-Jensen <achrii at fysik.dtu.dk>
Emne: Fwd: [gpaw-users] gw_skn and eshift in BSE




-------- Videresendt meddelelse --------
Subject:         [gpaw-users] gw_skn and eshift in BSE
Date:    Fri, 19 Jun 2020 07:40:51 +0200
From:    Jingjing SHAO via gpaw-users <gpaw-users at listserv.fysik.dtu.dk>
Reply-To:        Jingjing SHAO <jingjingshao0103 at gmail.com>
To:      gpaw-users <gpaw-users at listserv.fysik.dtu.dk>



Dear gpaw-users,

I have a bit of confusion with the eshift and gw_skn in the BSE module.
As eshift does already the correction of the eigenvalue of the KS-DFT
results, is it necessary to add the gw_skn wenn I only have calculated
the VB and CB in the G0W0?
I sometimes have the BSE spectrum appearing in the negative energy
region when the system has very little band gap... I am not sure where
it comes from and is it even possible? It would be really kind if you
could help to explain this observation from your perspective..
Thank you!

Best regards,
Jingjing


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