# NonLinearOptics code overview

This code is intended to be run after a full GW/BSE calculation and generates the UltraFast or 2-photon spectrum of a material by calculating the optical transition strength between the lowest excited exciton state (user inputed) and higher excited exciton states.

Danger

The NonLinearOptics ode should be considered alpha quality. It has been tested on only a few systems. Use at your own risk and validate and, if necessary, contact the developers.

### Required input files

• nonlinearoptics.inp : Input parameters.

• eigenvectors : The exciton wavefunctions in Bloch space.

• WFN_fi : Bloch wavefunctions on the unshifted fine grid.

• WFNq_fi : Bloch wavefunctions on the shifted fine grid. Only needed if velocity operator is used.

### Auxiliary Files

The files are output files from previous runs - used as input to speed up calculation

• vmtxel_nl: The interband optical matrix elements. This is different from the file vmtxel created by absorption in that it contains valence-valence and conduction-conduction matrix elements. Specify read_vmtxel_uf in the nonlinearoptics.inp file and files WFN_fi and WFNq_fi are not needed.