for the exact exciton energy

[Reza_Reza]

Dear Daniele,

I have calculated the excitonic energies and amplitude. The output file and energies are strange. For example, Band_v is 67 and Band_c is 66. It is completely inverse.
I also take into account the QP correction. However, I have a range of negative exciton energy.

I attached the report file,

Best,
Reza

[Daniele Varsano]

Dear Reza,
can you post the input and report files of your BSE calculation together with the o.exc*E_sort file?
Best,
Daniele

[Reza_Reza]

Dear Daniele,


I have attached all the requested files.
I also have a question: when I want to compute and plot the optical absorption, I should set BSEQptR= 1 | 26 and plot o-ev_3D_BSE.eps_q26_diago_bse. If i set it to 1 | 1, then the scale of optical absorption is very small (0.000002).
In that case, for exciton energy and wave function, I should set up the input file like this:
mpirun ypp -J 3D_BSE -e s -b 26 instead of mpirun ypp -J 3D_BSE e s 1

And for the amplitude:
excitons
amplitude
States= "2 - 2"
BSQindex= 26
Degen_Step= 0.000000


Best

[Daniele Varsano]

Dear Reza,

1) You are getting negative excitation energies because you have activated the coupling between resonant and antiresonant part of the BSE Hamiltonian.
In this case, Yambo prints both resonant (v-->c) and antireseonant (c-->v) excitations. You need to look at the positive value. But, is the coupling necessary in your case? Note that calculation with coupling are more expensive and in most of the case you do not need it, and you can stay with the Tamm-Dancoff approximation. You can check if the negative and positive part of the spectra are symmetric, which means that resonant and antiresonant coupling is negligible.

2) If you are interested in optical absorption, you need to calculate the q-->0 limit, i.e. BSEQptR= 1 | 1.
Finite q excitation are not excited by light but visible in electron energy loss experiments. In all the post-processing, you need to set BSQindex=1.

3) You get low strength because it seems you are simulating a 2D system. In this case the epsilon is not the right quantity to look at as it goes to zero for large supercell volume. The quantity to look at is the polarizability alpha.

Please note that most of these issues are explained in the tutorials

Best,
Daniele

[Jerry]

Dear Versano,
When I run the code of example/hBN-2D
I want to get exciton's amplitude by option

Code: Select all

ypp -e a
ypp

and I get an output file.
However I find nothing in the amplitude blanket. I want to know how to solve the problem.
Best,
Jerry

[Daniele Varsano]

Dear Jerry,
please check if in your report file (r_excitons_amplitude) you have the following warning:

Code: Select all

[WARNING] ndb.BS_diago not found

 [WARNING] BSE databases not found.

If this is the case be sure you have the ndb.BS_diago database in your SAVE directory, otherwise you need to redirect the calculation as:
ypp -J dirname

Be also sure you uncommented the flag WRbsWF in the previous BSE calculation.

Best,
Daniele