Yambo Community Forum

Dear all，

I am calculating the real-space distribution of excitons.
According to the tutorial https://wiki.yambo-code.eu/wiki/index.p ... e_excitons, I obtained the file o-2D_BSE.exc_qpt1_E_sorted, as follows:

E [ev] Strength Index
#
1.84255493 0.429079992E-4 1.00000000
1.84256220 0.559128763E-3 2.00000000
1.9177488 0.18821054E-10 3.0000000
2.03059983 0.120938203E-3 4.00000000
2.03060412 0.513948977 5.00000000
2.0495059 0.23753315E-10 6.0000000
2.07734680 0.899806619E-1 7.00000000
2.07735038 0.151320860 8.00000000
2.1278718 0.12041529E-09 9.0000000
2.15830755 1.00000000 10.0000000
2.15830898 0.263131678 11.0000000
2.16176772 0.346289752E-7 12.0000000
2.2123063 0.88778663E-11 13.000000
2.21337438 0.499288202E-3 14.0000000

I found that the intensity of the 10th exciton is the largest, so I plan to project the 10th exciton. In the input file, I have set the following:

excitons # [R] Excitonic properties
amplitude # [R] Amplitude
States= "1 - 11" # Index of the BS state(s)
BSQindex= 1 # Q-Index of the BS state(s)
Degen_Step= 0 eV # Maximum energy separation of two degenerate states

1、In the output file o-3D_BSE.exc_qpt1_weights_at_10 I obtained, I didn’t find any weight information. Does this mean the weight of the 10th exciton is less than 5%? I searched the forum and found that adding the parameter EXCITON_weight_threshold=0.01 would display excitons with weights greater than 1%, but it seems this parameter has no effect after being added.

2、How should I determine whether two excitons are degenerate? Is it based on the energy difference between the excitons?

excitons # [R] Excitonic properties
wavefunction # [R] Wavefunction
Format= "x" # Output format [(c)ube/(g)nuplot/(x)crysden]
Direction= "12" # [rlu] [1/2/3] for 1d or [12/13/23] for 2d [123] for 3D
FFTGvecs= 18001 RL # [FFT] Plane-waves
#NormToOne # Normalize to one the maximum value of the plot
States= "10 - 10" # Index of the BS state(s)
BSQindex= 1 # Q-Index of the BS state(s)
Degen_Step= 0 eV # Maximum energy separation of two degenerate states
% Cells
5 | 5 | 1 | # Number of cell repetitions in each direction (odd or 1)
%
% Hole
0.365034190E-7 | 3.87369299 | 23.7983723 | # [cc] Hole position in unit cell (positive)
%

3、Does a weight smaller than 5% affect the real-space projection of the exciton? When I continued following the tutorial, I used VESTA to check the projection and found no projection at all. Why is that?

I might have been a bit wordy, but I would be very happy with any suggestions！

Best,
sunxl

Dear Sunxl,

Firstly, be sure you wrote the eigenstate in the database, i.e. that the flag WRbsWF was present in the input file when solving the BSE.
If so, you can proceed with the steps on weights and exciton wave function.

If it is present, please post the report file of your calculations and we will have a look.

Degeneration: yambo recognize degenerate states based on energy difference (default is 0.01), but as you put Degen_Step= 0 eV each exciton is analysed separately.
The variable for the weight threshold is "Weight_treshold".

Best,

Daniele

Dear Daniele,

I uploaded my output file and looked at the WRbsWF flag.

Dear Sunxl,
can you post the report of the ypp run of the exciton wavefunction?
Is it possible that is just a matter of properly setting the VESTA isosurface?

Best,
Daniele

Dear Daniele,

In vesta,isosurface Best,
sunxl

Dear sunxl,

that's weird.
Is your BSE spectrum reasonable, or you have NaN?
Can you then check if your xsf contains NaN?
Can you also check if NaN are present in ./2D_WR_WC//ndb.BS_diago_Q1 file? You can convert in readable format using ncdump command, or you can do a grep NaN directly.

Check also if the problem persists changing the hole position, it seems it is located at the edge of the cell, can you try a different position?

Other comments, not related with the problem.

1. XTermKind is not recommended, it reduced that number of needed unoccupied bands, but it has a large prefactor and sometimes it is not very stable.
2. CUTGeo= "box z" is deprecated, use instead "slab z"

Best,
Daniele