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NonLinearOptics code
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  Version 2.0   (May, 2018)
  To be announced.

  Version 1.2   (Aug, 2016)
  F. H. da Jornada, J. Deslippe, D. Vigil-Fowler, J. I. Mustafa, T. Rangel,
  F. Bruneval, F. Liu, D. Y. Qiu, D. A. Strubbe, G. Samsonidze, J. Lischner.

  Version 1.0	(Aug, 2011) (Alpha)
  Version 0.5	(July, 2008) (Alpha)
  Version 0.2	J. Deslippe (2008) (Alpha)

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Description:

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.

WARNING - This code should be considered alpha quality. It has been tested
on only a few systems. Use at your own risk and validate.

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Required Input:

	nonlinearoptics.inp	Input parameters.  See detailed explanation below.
	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.

Additional Input:

	eqp.dat	This is not yet implemented (energies are read from A_svck) 
                        If it were to work, it would probably be: 
                        A list of quasiparticle energy corrections for the bands
			in WFN_fi and WFNq_fi. If absent the scissors operator from
			nonlinearoptics.inp is used.

Auxiliary Files: (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.

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	nonlinearoptics.inp	Please see example nonlinearoptics.inp in this directory
				for more complete options.

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Output Files:

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Tricks and hints:

