Band gap convergence

Concerns issues with computing quasiparticle corrections to the DFT eigenvalues - i.e., the self-energy within the GW approximation (-g n), or considering the Hartree-Fock exchange only (-x)

Moderators: Davide Sangalli, andrea.ferretti, myrta gruning, andrea marini, Daniele Varsano

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Daniele Varsano
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Re: Band gap convergence

Post by Daniele Varsano » Thu Jan 20, 2022 6:54 pm

Dear Laura,

usually it is not a problem, and it is a good strategy when converging wrt k points as gamma is present in all the centred grids. The only problem that can arise is that the wfs at gamma and at k points when the gap occurs have a different character so the parameter of convergence can be slightly different.

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Laura Caputo
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Joined: Sun Apr 11, 2021 3:02 pm

Re: Band gap convergence

Post by Laura Caputo » Fri Jan 21, 2022 10:38 am

Hello Daniele,

Just to be sure, I calculated the values again at the direct gap point at both 200 bands (as converged at Gamma) and at 350 bands. Compared to the Gamma point, the difference between the values changes more or less 2x10-3.
200 the gap is 1.9761 and at 350 is 1,9674. So, I guess, it still converged also at this point, right?
Laura Caputo
Ph.D. Student
Université Catholique de Louvain
https://uclouvain.be/fr/repertoires/laura.caputo

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Daniele Varsano
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Re: Band gap convergence

Post by Daniele Varsano » Fri Jan 21, 2022 10:46 am

Dear Laura,
from your numbers the difference is about 9 meV which is a reasonable threshold.

Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Laura Caputo
Posts: 88
Joined: Sun Apr 11, 2021 3:02 pm

Re: Band gap convergence

Post by Laura Caputo » Fri Jan 21, 2022 10:48 am

Dear Daniele,

Thanks. I checked also for other systems and the differences are even lower so I guess that I could take that as converged values. Thanks again,

Laura
Laura Caputo
Ph.D. Student
Université Catholique de Louvain
https://uclouvain.be/fr/repertoires/laura.caputo

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