how to compare with experiment results?

Deals with issues related to computation of optical spectra, in RPA (-o c) or by solving the Bethe-Salpeter equation (-o b). Includes local field effects, excitons, etc.

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Daniele Varsano
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Re: how to compare with experiment results?

Post by Daniele Varsano » Mon Apr 13, 2020 8:33 am

Dear Haseeb,

these questions concern the interpretation of the results and not the code usage. I think you should discuss them with your research group.
Just bear in mind that to be consistent you should consider the QP gap and BSE on top of QP energies (QP calculated either calculating GW correction or scissor operator when possibile to apply) and not KS energies.

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/

haseebphysics1
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Re: how to compare with experiment results?

Post by haseebphysics1 » Mon Apr 13, 2020 12:39 pm

Dear Daniele,

Code: Select all

I think you should discuss them with your research group
.

Sure, but I just want your comment on the correct procedure to calculate the exciton BE from Yambo! Because I think that I am getting two different results for the same thing!
QP gap and BSE on top of QP energies (QP calculated either calculating GW correction or scissor operator when possible to apply) and not KS energies.
Thanks, Daniele, in the attached plot, I have added the scissor corrections along the x-axis in computed results manually!

Regards,
Haseeb Ahmad
MS - Physics,
LUMS - Pakistan

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Daniele Varsano
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Re: how to compare with experiment results?

Post by Daniele Varsano » Mon Apr 13, 2020 12:51 pm

Dear Haseeb,
am getting two different results for the same thing!,
can you please repeat what are the two procedures and the respective results?

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/

haseebphysics1
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Re: how to compare with experiment results?

Post by haseebphysics1 » Mon Apr 13, 2020 1:21 pm

Dear Daniele,
can you please repeat what are the two procedures and the respective results?
Sure, to find the exciton BE

From the sorted_E ypp output file, "strongest bound exciton" has B.E equal to 2.32 - 2.0029 ~ 0.32 eV, while the absorption curve as attached in the last post shows the B.E greater than 1 eV! So, I was confused how to deduce the exciton B.E?

where, 2.32 is the KS-bandgap and 2.0029 is the first excitation in the ypp sorted file! I have not applied QP or scissor in BSE, instead I just added the sciccor value 0.48 eV in the absorption spectra manually! Therefore, the ypp is also reading the KS energies not corrected one.

Thanking you,
Haseeb Ahmad
MS - Physics,
LUMS - Pakistan

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Daniele Varsano
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Re: how to compare with experiment results?

Post by Daniele Varsano » Mon Apr 13, 2020 1:40 pm

Dear Haseebm
I do not know what you exactly mean with "strongest bound exciton". Note that your first exciton has very low intensity (~10^-3) so you cannot see it in the absorption plot. Each exciton has its own binding energy with respect to the gap. If excitations have energies larger than the gap they are not bound.

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/

haseebphysics1
Posts: 145
Joined: Sat Aug 17, 2019 2:48 pm

Re: how to compare with experiment results?

Post by haseebphysics1 » Thu Jul 16, 2020 8:23 pm

Dear Daniele, hope you are doing well!

Q1: I want to know why it does not make sense to do the spin-triplet BSE with spinors (Spin-Orbit coupling calculations)?

Q2: Is it possible to have a closer agreement with experiment with spin-triplet only (no exchange in BSE)? If it happens like so then, can we conclude that local field effects are not important in the material? or even making the results worse?

I thought that spin-singlet, being full BSE (under given approx.) and therefore should be closer to the experimental data?

Regards,
Haseeb Ahmad
MS - Physics,
LUMS - Pakistan

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Daniele Varsano
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Re: how to compare with experiment results?

Post by Daniele Varsano » Fri Jul 17, 2020 7:15 am

Dear Haseeb,
1) When dealing with spinor singlet and triplet excitation are not well defined by definition as singlet and triplet are not decoupled.
2) If you write the BSE for spin triplet you recognise that exchange part is not present in the excitonic Hamiltonian. When doing spin resolved calculation it can be shown that singlet and triplet excitation decouples see e.g. Rohlfing Louie PRB 62 4927 (200) Eq. 25 and relative discussion. When comparing with experiment it depends if you are looking at singlet or triplet excitations.

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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