Uniform sampling = no

[Sam-2025]

Dear all,


I would like to recheck the band gap vs Temperature for cubic BTO, using the following tutorial :https://wiki.yambo-code.eu/wiki/index.p ... n_Coupling
I ran scf and nscf using kpoint 6 6 6 and as mentioned in the tutorial I got the following "Q-points (IBZ) PW-formatted" = #20, then I ran phonon and DVSCF calculation(attached the input files), but after running ypp_ph, I got "Uniform sampling :no", , is it should be yes ? I mean when it is "no", how it can affect the result? Could you please let me know, what shall I do to reach a Uniform sampling?
Then
I tried to continue the tutorial and ran for different temperature, but my gap vs T, does not look correct! Also, could you please guide me , how should I choose the "QPkrange " variable in this section. BTO has indirect band gap from R to Gamma point and VBM = 20. Your guidance would be very useful for me. Thank you so much in advance.

<---> Q-points (IBZ) PW-formatted
0.000000000 0.000000000 0.000000000 1
0.000000000 0.000000000 0.166666642 1
0.000000000 0.000000000 0.333333284 1
0.000000000 0.000000000 -0.500000000 1
0.000000000 0.166666642 0.166666642 1
0.000000000 0.166666642 0.333333284 1
0.000000000 0.166666642 -0.500000000 1
0.000000000 0.333333284 0.333333284 1
0.000000000 0.333333284 -0.500000000 1
0.000000000 -0.500000000 -0.500000000 1
0.166666642 0.166666642 0.166666642 1
0.166666642 0.166666642 0.333333284 1
0.166666642 0.166666642 -0.500000000 1
0.166666642 0.333333284 0.333333284 1
0.166666642 0.333333284 -0.500000000 1
0.166666642 -0.500000000 -0.500000000 1
0.333333284 0.333333284 0.333333284 1
0.333333284 0.333333284 -0.500000000 1
0.333333284 -0.500000000 -0.500000000 1
-0.500000000 -0.500000000 -0.500000000 1
<---> [08] Timing Overview

[Sam-2025]

scf, nscf files also attached for your consideration.

[Sam-2025]

Dear all,

no one had this issue before? any guidance would be very useful. Thank you so much in advance.

[palful]

Dear Samira,

Indeed, you should get "Uniform sampling = 'yes'", which basically means that Yambo is recognising the electronic momentum k-grid and the momentum-transfer q-grid to be the same.

According to the tutorial, you should have inverted the signs of the momentum coordinates given by ypp before running the phonon calculations: did you do so? If you don't get uniform sampling, I am afraid the calculation would be possible only at the gamma point.

Assuming that you're able to get the uniform sampling, in order to calculate the indirect band gap correction, you can specify something like this:
%QPkrange
1|1|21|21| # Correction to the conduction band 21 at gamma
iR|iR|20|20| # Correction to the valence band 20. iR here is the index of the R point
%

The yambo interface for electron-phonon calculations is quite dated, and we are currently working on a better interface. I expect that in the future the tutorial you are following will change, but for now, try to follow the suggestions above!

Best,
Fulvio

[Sam-2025]

Dr. Fulvio Paleari

Thank you so much for your valuable guidance and help.
I used kpoint 8 8 8 and got the uniform sampling = yes
then ran a phonon and dvscf calculation and tried to check band gap vc T (https://wiki.yambo-code.eu/wiki/index.p ... n_Coupling)
(in and out files attached for your consideration)
but the band gap vc T is not changing which is not correct.
Could you please guide me , where I am doing wrong. I sincerely appreciate your time and help.

100 1.635844
200 1.635844
300 1.635844
400 1.635844
500 1.635844

[Sam-2025]

some of other input files for your consideration.

[Sam-2025]

nscf . out file is also attached. Thank you so much.
BTO has indirect and direct gap
it's gap is around 3.2 eV using the GW and experimental but with standard DFT is around 1.7