GW vs GGA band gap

Run-time issues concerning Yambo that are not covered in the above forums.

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lamia
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Joined: Mon Dec 05, 2016 5:09 pm

Re: GW vs GGA band gap

Post by lamia » Thu Jul 16, 2020 1:42 pm

Dear Daniele,

What's his name and where could I find it ?

Best,

Lamia
Taouil Lamia
University of Mohammed V
Faculty of science - Rabat
Morocco

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Daniele Varsano
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Re: GW vs GGA band gap

Post by Daniele Varsano » Thu Jul 16, 2020 2:14 pm

Dear Taouil,
these are the files containing the band output (o.bands*), where are you looking the gap?
Note that the gap reported in the report indicated with X " [X]Indirect Gaps" is always the KS gap, so you cannot see the GW gap from there, but you need to inspect the bands output.

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/

lamia
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Re: GW vs GGA band gap

Post by lamia » Thu Jul 16, 2020 2:28 pm

Dear Daniele,

Thank you so much for your comment. Could you give me more details about it, please?


Regards,
Lamia
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Taouil Lamia
University of Mohammed V
Faculty of science - Rabat
Morocco

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Daniele Varsano
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Re: GW vs GGA band gap

Post by Daniele Varsano » Thu Jul 16, 2020 2:50 pm

Dear Lamia,
looking at your bands (see plot) you can recognise that the direct gap at gamma is about 2.58eV which correspond to the GW gap you find in the o.qp file:

Code: Select all

1.000000     4.000000    -1.466253    -1.278971     0.187282     0.743058
1.000000     5.000000     0.300217     1.299281     0.999064     0.762584
So, the GW corrections are taken into account.

Best,
Daniele
GWbands.png
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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/

lamia
Posts: 45
Joined: Mon Dec 05, 2016 5:09 pm

Re: GW vs GGA band gap

Post by lamia » Thu Jul 16, 2020 3:46 pm

Dear Daniele ,

how could I read the value of band gap from o.qp?
and actually my structure has an indirect band gap with 2.43 eV wich doesn't correspond to what was found? could you suggest any solution for that?

best,
Lamia
Taouil Lamia
University of Mohammed V
Faculty of science - Rabat
Morocco

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Daniele Varsano
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Re: GW vs GGA band gap

Post by Daniele Varsano » Thu Jul 16, 2020 3:56 pm

Dear Lamia,
in the o.qp you have the KS energies and their GW correction for all the points you have calculated.
From that you can extract your indirect band gap.
What is the 2.43 eV value? If this is the experimental gap, there could be many reason why the GW gap is different:
1. Convergence parameters
2. G0W0 starting DFT ground state i.e. absence of self consistency
3. strongly correlated system...

and so on...

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/

lamia
Posts: 45
Joined: Mon Dec 05, 2016 5:09 pm

Re: GW vs GGA band gap

Post by lamia » Thu Jul 16, 2020 4:25 pm

Dear Daniele,

Could you explain more the second and third points, please?
and witch parameters correspond to them?
ps: The system is 2D

regards,

Lamia
Taouil Lamia
University of Mohammed V
Faculty of science - Rabat
Morocco

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Daniele Varsano
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Re: GW vs GGA band gap

Post by Daniele Varsano » Thu Jul 16, 2020 4:33 pm

Dear Lamia,

I just wanted to say that there could be many reason why GW does not predict the experimental gap of a materials.
It is a powerful theory but it also has limitations: see e.g.
https://onlinelibrary.wiley.com/doi/abs ... /wcms.1344

Now, I do not know what system you are dealing with, so I cannot say much. Maybe you can have a look if there are other GW calculations on your system in the literature and compare with them. Of course, careful converge tests are mandatory.

Best,
Daniele

PS: if the system is 2D there is a strong dependency on the vacuum size and you are surely underestimating your gap. In order to mitigate this issue you should use truncated coulomb potential. You can have a look to the yambo tutorial on how to use them.
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/

lamia
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Joined: Mon Dec 05, 2016 5:09 pm

Re: GW vs GGA band gap

Post by lamia » Thu Jul 16, 2020 4:37 pm

Dear Daniele ,

The system is SnC -2D

best,

Lamia
Taouil Lamia
University of Mohammed V
Faculty of science - Rabat
Morocco

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Daniele Varsano
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Re: GW vs GGA band gap

Post by Daniele Varsano » Thu Jul 16, 2020 4:40 pm

OK, but I think it is your job to understand what is going on.
Please have a look to my comment above about treating the interaction in 2D system, you need to use truncated coulomb potential to avoid spurious interaction among images.

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/

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