No Absorption with BSE calculation on monolayer MoS2

Deals with issues related to computation of optical spectra, solving the Bethe-Salpeter equation.

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

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Daniele Varsano
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Re: No Absorption with BSE calculation on monolayer MoS2

Post by Daniele Varsano » Tue Mar 19, 2024 4:24 pm

Dear Reza,

as you can see the IP spectrum onset correspond to the gap you posted before, it seems to me perfectly reasonable.
About the q=0 limit, this is the optical spectrum and if you are interested in optic you can set in input:

Code: Select all

% BSEQptR
   1 | 1 |                           # [BSK] Transferred momenta range
%
and this will save you a lot of computer time.
You calculated instead the response for all the 26 q points, and you should have 26 outputs, besides iq=1 (q0 limit) the others does not describe optical excitations but excitations with transferred finite momentum, which are observable e.g. in EELS experiments.

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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Re: No Absorption with BSE calculation on monolayer MoS2

Post by Reza_Reza » Wed Mar 20, 2024 2:25 pm

Dear Daniele,

Thank you very much for your response.
I only have ambiguity. I don't understand the concept of iq=1 (q0 limit) here.
Also, what is the difference between the spectra when I plot u 1:2 and u 1:4 ?

Best,

Reza
Reza - Postdoc
CNRS

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Daniele Varsano
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Re: No Absorption with BSE calculation on monolayer MoS2

Post by Daniele Varsano » Wed Mar 20, 2024 5:49 pm

Dear Reza,

In the BSE output file, you have the BSE results (column 2 and 3) and also the Independent Particle spectra (column 3 and 4).
In Yambo each q points is associated to an index, the list of q points and corresponding indexes are reported in the report file. The q=0,0,0 is always index 1.
If you want to calculate optics, then you want to calculate the response at q=0 limit (vertical excitations) so you need to set iq=1.
You can have a look at the Yambo cheatsheet (slide 11a):
https://www.yambo-code.eu/wiki/images/c ... et-5.0.pdf

If these concepts are not crystal clear, I suggest you to go step-by-step looking to the tutorials. In the Yambo wiki you can find several og them.
For instance, here: https://www.yambo-code.eu/wiki/index.ph ... #Tutorials
you can find a tour from Independent particle calculations, GW and Bethe Salpeter.

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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