questions about the results of BSE calculations?

Deals with issues related to computation of optical spectra in reciprocal space: RPA, TDDFT, local field effects.

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Dean
Posts: 98
Joined: Thu Oct 10, 2019 7:03 am

questions about the results of BSE calculations?

Post by Dean » Thu Sep 10, 2020 8:36 am

Dear developers,
1. After doing BSE calculations with Diago solver, I want to analyze the exciton oscillator strenght and amplitude and got the file o-2D.exc_weights_at_1, which shows :
# Electron-Hole pairs that contributes to Excitonic State 1 more than 5%
#
# K-point [iku] Weight
# 0.26667 0.46667 0.00000 0.09283
# 0.333333 0.500000 0.000000 1.000000
#
# Band_V Band_C K ibz Symm. Weight Energy
#
21.00000 22.00000 27.00000 2.00000 0.90921 2.06548
21.00000 22.00000 27.00000 1.00000 0.90919 2.06548
My question is why it show two same kpoints (ibz: 27) contributing to this Excitonic State? but two different k-points in iku Points coordinates?
2. After doing BSE calculations with Inversion solver, the file o-2D-.eps_q1_inv_bse shows 7 columns. What does each column stand for?
With regards,
Thanks.
Dr. Yimin Ding
Soochow University, China.

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Daniele Varsano
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Re: questions about the results of BSE calculations?

Post by Daniele Varsano » Thu Sep 10, 2020 9:11 am

Dear Yimin,
My question is why it show two same kpoints (ibz: 27) contributing to this Excitonic State?
These are not the *same* k point in the BZ, but two k points related by symmetry (4th column)!
The matrix relating the two k points is listed in the report file where the symmetries are indexed (here 1 is the identity, the index 2 is listed in the report).
but two different k-points in iku Points coordinates?
This is a different list, as in the K-point analysis all the v-> c transitions our a given k points are summed up.
o-2D-.eps_q1_inv_bse shows 7 columns. What does each column stand for?
# E/ev[1] EPS-Im[2] EPS-Re[3] EPSo-Im[4] EPSo-Re[5] EPS`-Im[6] EPS`-Re[7]
energy, imaging/real part of eps , imag/real part of non interacting eps , imag/real part of eps for the iteration before convergence

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/

Dean
Posts: 98
Joined: Thu Oct 10, 2019 7:03 am

Re: questions about the results of BSE calculations?

Post by Dean » Fri Sep 11, 2020 2:02 am

Dear Daniele,
Thanks very much.
Dr. Yimin Ding
Soochow University, China.

pyadav
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Re: questions about the results of BSE calculations?

Post by pyadav » Thu Mar 10, 2022 4:07 pm

Dear team Yambo,

I have done a BSE calculation and sorted the exciton eigenvalues. I have also produced the 'amplitudes' and 'weights' files'. From the absorption plot and the sorted eigenvalues, I have my first exciton peak at 2.35 eV but from the weight file, the last column shows the transition energy =1.99 eV for the same exciton.

I seek someone's response to clarify the difference in these energies here. Why the last column of the weight file has a different energy than the exciton resonance energy.
Please see the attachment for more details.

It will be also very helpful if I may know how can I plot the exciton WF in the momentum space using the weight file.

Thank you,
Pushpendra
You do not have the required permissions to view the files attached to this post.
Pushpendra Yadav
Ph.D. Research Scholar
Quantum Transport and Theory Group
Department of Physics
Indian Instittute of Technology Kanpur, India.

https://sites.google.com/site/amitkag1/

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Daniele Varsano
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Re: questions about the results of BSE calculations?

Post by Daniele Varsano » Mon Mar 14, 2022 2:25 pm

Dear Pushpendra,

the weight file provides you the Kohn-Sham transition involved in a given exciton and the last column it is its KS transition energy (Ec-Ev). Note it is the KS and not the QP energy and it is its independent particle contribution so it should not be equal to the exciton energy.

About excitons in K space you can have a look to this thread:
viewtopic.php?t=1656

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/

pyadav
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Re: questions about the results of BSE calculations?

Post by pyadav » Mon Mar 14, 2022 7:00 pm

Dear Daniele,

Thank you so much for the reply and the clarifications.

I have another query about the same file-

Code: Select all

 Electron-Hole pairs that contribute to Excitonic State 56 for iq=1 more than  5.000000%
#
#                       K-point [iku]                         Weight
# :    0.00000000         0.00000000         0.00000000        0.176415621   
# :    0.00000000        0.833333333E-1      0.00000000        0.595666140   
# :   0.833333333E-1     0.125000000         0.00000000        0.894073849E-1
# :   0.250000000        0.458333333         0.00000000         1.00000000   
#    
#    Band_V             Band_C             Kv-q ibz           Symm_kv            Kc ibz             Symm_kc            Weight             Energy
#    
   41.0000000      44.0000000      18.0000000      7.00000000      18.0000000      7.00000000     0.161323024      2.65908217   
   .
   . 
   42.0000000      45.0000000      1.00000000      1.00000000      1.00000000      1.00000000     0.720630997E-1   2.82974004   
   41.0000000      46.0000000      1.00000000      1.00000000      1.00000000      1.00000000     0.720630310E-1   2.82974004   
   41.0000000      45.0000000      1.00000000      1.00000000      1.00000000      1.00000000     0.713252177E-1   2.82974004   
   42.0000000      46.0000000      1.00000000      1.00000000      1.00000000      1.00000000     0.713251544E-1   2.82974004   
In this output section, I understand that the exciton is appearing between the
: 41-44, at K [18]: 0.25 0.33 0.00 [rlu],
: 42-45, at K [18]: 0.00 0.00 0.00 [rlu],
and so on, via direct transitions. I want to know what does the following lines say?

Code: Select all

#                       K-point [iku]                         Weight
# :    0.00000000         0.00000000         0.00000000        0.176415621   
# :    0.00000000        0.833333333E-1      0.00000000        0.595666140   
# :   0.833333333E-1     0.125000000         0.00000000        0.894073849E-1
# :   0.250000000        0.458333333         0.00000000         1.00000000   
 
Best,
Pushpendra
Pushpendra Yadav
Ph.D. Research Scholar
Quantum Transport and Theory Group
Department of Physics
Indian Instittute of Technology Kanpur, India.

https://sites.google.com/site/amitkag1/

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Daniele Varsano
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Re: questions about the results of BSE calculations?

Post by Daniele Varsano » Tue Mar 15, 2022 11:31 am

These provide the same information integrated on bands. So it is the weight of the vertical transition at a given k point where the contribution on different bands has been summed up.
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/

pyadav
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Re: questions about the results of BSE calculations?

Post by pyadav » Tue Mar 15, 2022 12:22 pm

Dear Daniele,

Thank you for the response.

A small confusion here for me is as follow-

Since from the different band transitions, we see that there are only two irreducible k-points (18 and 1) are involved which correspond to (0.25, 0.33, 0.00 and 0.00, 0.00, 0.00) respectively. I'm unable to understand why there are four k-points

Code: Select all

#                       K-point [iku]                         Weight
# :    0.00000000         0.00000000         0.00000000        0.176415621   
# :    0.00000000        0.833333333E-1      0.00000000        0.595666140   
# :   0.833333333E-1     0.125000000         0.00000000        0.894073849E-1
# :   0.250000000        0.458333333         0.00000000         1.00000000   
and only $\Gamma$ point is consistent but not the other three k-points with the irreducible rep. k-points.

This may be trivial for you but I'm just not able to catch the correspondence between the commented and uncommented k-points.

Apart from this, what is the difference between strength and weight? does strength mean oscillator strength?
Any help would be much appreciated.




Thank you,
Pushpendra.
Pushpendra Yadav
Ph.D. Research Scholar
Quantum Transport and Theory Group
Department of Physics
Indian Instittute of Technology Kanpur, India.

https://sites.google.com/site/amitkag1/

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Daniele Varsano
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Re: questions about the results of BSE calculations?

Post by Daniele Varsano » Wed Mar 16, 2022 4:10 pm

Dear Pushpendra,

the weights are the modulus square of the BSE eigenstate |A^i_kcv|^2, while the strengths involve the dipole moments and tells you if the exciton is bright or not.

Note that \sum_|A^ikcv|^=1 and it happens it is not your case. This means you have degenerate exciotns that have been merged. Moreover in the output are reported only the weight > threshold (5%).

In order to avoid confusion you can set the following variables in the ypp input file:

Code: Select all

Weight_treshold=0.01 eV
e.g to have the transition larger than 1% or 0eV to have all the transition.
and:

Code: Select all

Degen_Step=   0.0  eV    # Maximum energy separation of two degenerate states
to consider just one single exciton.

Please note also that to each k point in the IBZ is associated a symmetry to bring it in the BZ, in your case is the symm number 7, in the report you can find the associated rotation matrix.

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/

pyadav
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Joined: Thu Nov 26, 2020 2:56 pm
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Re: questions about the results of BSE calculations?

Post by pyadav » Thu Mar 17, 2022 6:27 pm

Dear Prof. Daniele,

Thank you so much for your response and the clarifications. I'm really thankful to you for your kind help.

Best wishes,
Pushpendra.
Pushpendra Yadav
Ph.D. Research Scholar
Quantum Transport and Theory Group
Department of Physics
Indian Instittute of Technology Kanpur, India.

https://sites.google.com/site/amitkag1/

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