Yambo Community Forum

Posted: **Sat Feb 01, 2025 5:09 pm**

Dear all,

Recently, I have been using TD-BSE method to calculate the shg of two-dimensional materials. When I set the EFEILD1 to 1,0,0 (that is, the x direction), I can get two components, shg_yxx and shg_xxx, when the calculation is completed. Because two-dimensional materials cannot calculate the out-of-plane components, we will ignore shg_zxx first. For my materials, yxx is a non-zero component, while xxx is a zero component according to the symmetry requirements, but when I finish the calculation, I find that the maximum value of yxx is 1000 pm/V, while the maximum value of xxx is 200 pm/V, why the obtained xxx is also a non-zero component? So is the yxx non-zero component data that I get still reliable?

Best,
sunxl

Posted: **Mon Feb 03, 2025 3:36 pm**

Dear sunxl

did you get similar results also the the independent particle level?

another question, did you remove symmetries using ypp_nl? (double precision is important)

best
Claudio

Posted: **Mon Feb 03, 2025 4:38 pm**

Dear Claudio,

Yes, when I used IPA in my previous test, it also produced non-zero results. I made sure to remove the symmetry, followed the tutorial to set EFEILD1 to 1,0,0, and uncommented the symmetry of the time inversion using ypp_rt. Don't ypp_rt and ypp_nl have the same effect when removing symmetry?

fixsyms # [R] Remove symmetries not consistent with an external perturbation
% Efield1
1.000000 | 0.000000 | 0.000000 | # First external Electric Field
%
% Efield2
0.000000 | 0.000000 | 0.000000 | # Additional external Electric Field
%
BField= 0.000000 T # [MAG] Magnetic field modulus
Bpsi= 0.000000 deg # [MAG] Magnetic field psi angle [degree]
Btheta= 0.000000 deg # [MAG] Magnetic field theta angle [degree]
#RmAllSymm # Remove all symmetries
RmTimeRev # Remove Time Reversal
#RmSpaceInv # Remove Spatial Inversion
#KeepKGrid # Do not expand the k-grid

Best,
sunxl

Posted: **Thu Apr 10, 2025 11:09 am**

Dear sunxl

did you are able to get correct symmetries on a simple system like hBN? you can find inputs here:
https://wiki.yambo-code.eu/wiki/index.p ... X,_TD-BSE)

another question: are you sure atoms in your DFT input are not rotated in the sense that x corresponds to y?

finally comments: from real-time simulations there is always a small signal in all direction but it is order of magnitude smaller when it is forbidden by symmetries

let us know
claudio

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