Angular dependence of non-linear response

In this tutorial we show how to calculate angular dependence of non-linear response using Yambo. (This tutorial requires Yambo >= 5.3)
We consider as example a monolayer hBN, DFT input files can be downloaded here: hBN-2D-RT.tar.gz
In this tutorial we suppose you already know how to calculate non-linear reponse with Yambo,
if this is not the case please have a look to the tutorials: Non linear response

Run DFT calculation, import the wave-function and run the setup. Then

Remove symmetries

We remove all symmetries Using the command ypp_nl -y

fixsyms                          
# [R] Remove symmetries not consistent with an external perturbation
% Efield1
  0.000000 | 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 Time Reversal

notice that in principle one could remove only the symmetries of the rotation plane plus the TimeReversal.
Then go in the FixSymm folder and run the setup again

Run a standard SHG calculation

We first run a SHG calculation using the input

nloptics                         # [R] Non-linear spectroscopy 
% NLBands
   3 | 6 |                           # [NL] Bands range
%
NLverbosity= "high"              # [NL] Verbosity level (low | high)
NLtime=-1.000000           fs    # [NL] Simulation Time
NLintegrator= "INVINT"           # [NL] Integrator ("EULEREXP/RK2/RK4/RK2EXP/HEUN/INVINT/CRANKNIC")
NLCorrelation= "IPA"             # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/LRW/JGM/SEX")
NLLrcAlpha= 0.000000             # [NL] Long Range Correction
% NLEnRange
   1.000000 |6.000000 |         eV    # [NL] Energy range 
%
NLEnSteps=  160                    # [NL] Energy steps
NLDamping= 0.200000        eV    # [NL] Damping (or dephasing)
RADLifeTime=-1.000000      fs    # [RT] Radiative life-time (if negative Yambo sets it equal to Phase_LifeTime in NL)
% Field1_Freq
 0.100000 | 0.100000 |         eV    # [RT Field1] Frequency
%
Field1_NFreqs= 1                 # [RT Field1] Frequency
Field1_Int=  1000.00       kWLm2 # [RT Field1] Intensity
Field1_Width= 0.000000     fs    # [RT Field1] Width
Field1_kind= "SOFTSIN"           # [RT Field1] Kind(SIN|COS|RES|ANTIRES|GAUSS|DELTA|QSSIN)
Field1_pol= "linear"             # [RT Field1] Pol(linear|circular)
% Field1_Dir
  0.000000 | 1.000000 | 0.000000 |        # [RT Field1] Versor
%
Field1_Tstart= 0.010000    fs    # [RT Field1] Initial Time

from this run we get the X²_yyy(ω) spectrum:

where we put an arrow at 2.45 eV, the frequency where we want to study the angula dependence of the SHG. Notice that this specrum is not a converged one due to the small k-point grid and the reduced number of bands.

Run SHG at different angles

Using the same command yambo_nl -u n to generate the input at different angles:

nloptics                         # [R] Non-linear spectroscopy
% NLBands
   3 |  6 |                           # [NL] Bands range
%
NLverbosity= "high"              # [NL] Verbosity level (low | high)
NLtime=-1.000000           fs    # [NL] Simulation Time
NLintegrator= "INVINT"           # [NL] Integrator ("EULEREXP/RK2/RK4/RK2EXP/HEUN/INVINT/CRANKNIC")
NLCorrelation= "IPA"             # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/LRW/JGM/SEX")
NLLrcAlpha= 0.000000             # [NL] Long Range Correction
% NLEnRange
-1.000000 |-1.000000 |         eV    # [NL] Energy range (for loop on frequencies NLEnSteps/=0
%
NLEnSteps=0                      # [NL] Energy steps for the loop on frequencies
% NLrotaxis
  0.000000 | 0.000000 | 1.000000 |        # [NL] Rotation axis (for the loop on angles NLAngSteps/=0)
%
NLAngSteps= 80                     # [NL] Angular steps (if NLAngSteps/=0 field versor will be ignored)
NLDamping= 0.200000        eV    # [NL] Damping (or dephasing)
RADLifeTime=-1.000000      fs    # [RT] Radiative life-time (if negative Yambo sets it equal to Phase_LifeTime in NL)
% Field1_Freq
  2.500000 | 2.500000 |         eV    # [RT Field1] Frequency
%
Field1_NFreqs= 1                 # [RT Field1] Frequency
Field1_Int=  1000.00       kWLm2 # [RT Field1] Intensity 
Field1_Width= 0.000000     fs    # [RT Field1] Width
Field1_kind= "SOFTSIN"           # [RT Field1] Kind(SIN|SOFTSIN| see more on src/modules/mod_fields.F)
Field1_pol= "linear"             # [RT Field1] Pol(linear|circular)
% Field1_Dir
  0.000000 | 1.000000 | 0.000000 |        # [RT Field1] Versor
%
Field1_Tstart= 0.010000    fs    # [RT Field1] Initial Time

In this input we ask to Yambo to rotate the laser vector around the z-axis (NLrotaxis = 0.000000 | 0.000000 | 1.000000 ),
starting from the vector in the plane Field1_Dir=0.000000 | 1.000000 | 0.000000.
Thefore 0-degree means y-direction, while 90-degree is equivalent to x-direction. If you run the code Yambo will generate 80 angles (NLAngSteps) between 0 and 360 degree and run the calculations.
Result can be analysed with the same tools of SHG, namely the command ypp_nl -u n