Yambo Community Forum

Hello,

I have some doubts regarding the definition of exciton state indexes. After running a BSE calculation and sorting the osc. strenghts with "ypp -J 2D_WR_WC -e s 1, it generates for example a file "o-bse_out.exc_I_sorted" with the osc. strenghts sorted by intensity, also printing some indexes. In my case for example I obtain:

#
# E [ev] Strength Index
#
.
.
.
0.89355 0.70850 20.00000
0.89519 0.66522 21.00000
0.89544 0.54987 22.00000
.
.
.

Now, I want to analyse this exciton at 0.89eV using single-particle states. If I run "ypp -F ypp_AMPL.in -J 2D_WR_WC -e a 1 " using:

excitons # [R] Excitons
amplitude # [R] Amplitude
States= "20 - 22" # Index of the BS state(s)
dos # [R] DOS
DipWeight # Weight the contribution to the exciton WFs with the dipoles
Degen_Step= 0.01 eV # Maximum energy separation of two degenerate states

It generates some outputs, where inside "o-bse_out.exc_weights_at_20", "o-bse_out.exc_weights_at_21" and "o-bse_out.exc_weights_at_22" the code prints me the following output:

# Band_V Band_C K ibz Symm. Weight Energy
#
78.00000 83.00000 1.00000 1.00000 0.60114 1.15165
77.00000 84.00000 1.00000 1.00000 0.59925 1.15165
75.00000 83.00000 1.00000 1.00000 0.54685 1.15165
76.00000 84.00000 1.00000 1.00000 0.54129 1.15165
75.00000 84.00000 1.00000 1.00000 0.54100 1.15165
76.00000 83.00000 1.00000 1.00000 0.53360 1.15165
77.00000 83.00000 1.00000 1.00000 0.48370 1.15165
78.00000 84.00000 1.00000 1.00000 0.48299 1.15165

But the reported energies are not the same from the previous case. So I wonder if the "Index" of the first output is not related to the "States= "20 - 22"" flag on the second input. I also tried to change states to 1 - 20 to check if the code would order the excitons by energy, but analysis "o-bse_out.exc_weights_at_1" the energy reported is 0.8723, which is also not the same from the first case. So my question is how can I do the above decomposition exact for the states 20 to 22 with energies 0.89. even using a "Degen_Step= 0.01 eV " I cannot find this exciton at 0.89eV in any of the weights files.

Regards,

Dear Bruno,

first, note that with the Degen_step you are using the excitons 20-22 are seen as degenerate and a linear combination of them is considered. If you want to analyze each of them you should set the degeneracy step to 0.0 eV.

The energies reported in the *weights file are not the exciton energies but the KS energy (not QP corrected) differences between the conduction and valence band participating in the excitation with the reported weight.

Best,
Daniele