Yambo Community Forum

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

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

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

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

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

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

Dear Daniele,

I am calculating excitation dispersions for a system, both in the triplet and singlet cases.
For the triplet calculation, I set BSENGexx = 0 Ry.

I would like to compare the excitation dispersion for a specific exciton. For example, in the singlet case, the first bright exciton has index = 1, whereas in the triplet case it has index = 3.
Are these excitons directly comparable, or should I instead focus on comparing excitons with the same index in both the triplet and singlet calculations?

I have attached the results for both singlet and triplet cases.

For Triplet:
# Maximum Residual Value = 0.22825E+01
#
# E [ev] Strength Index
#
0.796852946 1.00000000 3.00000000
0.600751817 0.245987400 1.00000000

For the Singlet:# Maximum Residual Value = 0.22204E+01
#
# E [ev] Strength Index
#
0.947745442 1.00000000 1.00000000
1.35001147 0.206785411 4.00000000


Best,
Reza

Dear Reza,

apologize for the delay in answering, in the last month we have experienced problems with the forum website that have been solved only now.
Note that the intensity for the triplet states is not meaningful as these transitions are spin-forbidden, hence they are all dark.
I would compare the lowest excitation energies for both cases.

Best,

Daniele