Yambo Community Forum

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: 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

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

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