Sum frequency generation

In this tutorial we will show how to calculate Sum Frequency Generation(SFG) and also Difference Frequency Generation (DFG) in bulk materials.
We suppose you are already familiar with the non-linear response using the Yambo code. If it is not the case please study the previous tutorials:
Linear response using Dynamical Berry Phase and Real time approach to non-linear response (SHG).

Removing symmetries

In this tutorial we will calculate the SFG along when both fields are in the 'y' direction, therefore we remove symmetries not compatible with an external field along this direction, with the command ypp_nl -y:

fixsyms                          # [R] Remove symmetries not consistent with an 
external perturbation
% Efield1
  0.000000 | 1.000000 | 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