# ABINIT, optic input variables:

## List and description.

This document lists and provides the description of the name (keywords) of the "optic" input variables to be used in the input file for the OPTIC executable of the ABINIT package.

##### Copyright (C) 1998-2017 ABINIT group (DCA,XG,RC,YG,FJ)

### Content of the file : alphabetical list of "optic" variables.

A.
B. broadening
C.
D. ddkfile domega
E.
F.
G.
H.
I.
J.
K.
L. lin_comp
M. maxomega
N. nonlin_comp num_lin_comp num_nonlin_comp
O.
P.
Q.
R.
S. scissor
T. tolerance
U.
V.
W. wfkfile

broadening
Mnemonics: BROADENING
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: real
Default is 1.d-3 Ha

In Eq. 46 of Ref. 1, it is clear that when ever wnm(k) is equal to w, there is a resonance. Numerically this would lead to an infinity. In order to avoid this one could do two things. You could change the sum over k-points to integration and then use linear tetrahedron method (see Ref. 2 for details). Another way to get around the problem is, like we do in the present case, avoid this singularity by adding a small complex number to the denominator. This prevents the denominator from ever going to 0 and acts as a broadening to the spectrum. The broadening should not be too large as this would wash out the features in the spectrum.

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ddkfile
Mnemonics: DDK FILE
Executable: optic
Mentioned in topic: Optic.
Rarely used: [0/6] in optic tests, [0/2] in tuto optic tests.
Variable type: string
No default (Comment: no default)

Specify the filename that has been produced by the preparatory Abinit run. This file must contain the matrix elements of the d/dk operator along direction X. It must not contain the first-order wavefunctions and may be generated using prtwf 3.
You should make sure that the number of bands, of spin channels and of k-points are the same in all the files.

use as string with the filename: ddkfile_X, where X is the file number.

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domega
Mnemonics: Delta OMEGA
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: real
Default is 1.d-3 Ha

The step and maximum sets your energy grid for the calculation using the formula number of energy mesh points=maximum/step (zero excluded). So in order to capture more features you can decrease the step size to get a finer energy grid. In order to go to higher frequency, increase the maximum.

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lin_comp
Mnemonics: LINear COMPonents
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: integer(['num_lin_comp'])
Default is 0

This tells which component of the dielectric tensor you want to calculate. These numbers are called a and b Eqs. 46 in Ref. 1. 1 2 3 represent x y and z respectively. For example 11 would be xx and 32 would mean zy.

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maxomega
Mnemonics: MAXimum value of OMEGA
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: real
Default is 1 Ha

The step and maximum sets your energy grid for the calculation using the formula number of energy mesh points=maximum/step (zero excluded). So in order to capture more features you can decrease the step size to get a finer energy grid. In order to go to higher frequency, increase the maximum.

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nonlin_comp
Mnemonics: NON-LINear COMPonents
Executable: optic
Mentioned in topic: Optic.
Moderately used: [3/6] in optic tests, [1/2] in tuto optic tests. Test list: {tutorespfn:[optic_2],v4:[57],v7:[48]}.
Variable type: integer(['num_nonlin_comp'])
Default is 0

This tells which component of the dielectric tensor you want to calculate. These numbers are called a, b and c in Ref. 1. 1 2 3 represent x y and z respectively. For example 111 would be xxx and 321 would mean zyx.

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num_lin_comp
Mnemonics: NUMber of LINear COMPonents
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: integer
Default is 0

How many components out of 9 of the linear optical dielectric tensor do you want to calculate. Most of these are either equal or zero depending upon the symmetry of the material (for detail see Ref. 3).
Note that the directions are along the Cartesian axis.

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num_nonlin_comp
Mnemonics: NUMber of NON-LINear COMPonents
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: integer
Default is 0

How many components out of 27 of the non-linear optical dielectric tensor do you want to calculate. Most of these are either equal or zero depending upon the symmetry of the material (for detail see Ref. 3).
Note that the directions are along the Cartesian axis.

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scissor
Mnemonics: SCISSOR operator
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: real
Default is 0.0 (Comment: in Ha)

LDA/GGA are well known to underestimate the band-gap by up to 100%. In order to get the optical spectrum and make a realistic comparison with experiments one needs to correct for this. This can be achieved in two ways. The scissors shift is normally chosen to be the difference between the experimental and theoretical band-gap and is used to shift the conduction bands only. Another way in which you do not have to rely on experimental data is to determine the self energy using the GW approach. In this case the opening of the gap due to the GW correction can be used as scissor shift.

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tolerance
Mnemonics: TOLERANCE
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: real
Default is 1.d-3 Ha

When energy denominators are smaller than tolerance, the term is discarded from the sum.

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wfkfile
Mnemonics: WaveFunction K FILE
Executable: optic
Mentioned in topic: Optic.
Very frequently used: [6/6] in optic tests, [2/2] in tuto optic tests. Test list: {tutorespfn:[optic_2,optic_4],v4:[57],v67mbpt:[53],v7:[42,48]}.
Variable type: string
No default (Comment: no default)

Specify the filename that has been produced by the preparatory Abinit run. This file must contain the matrix elements of the d/dk operator along direction X. It must not contain the first-order wavefunctions and may be generated using prtwf 3.
You should make sure that the number of bands, of spin channels and of k-points are the same in all the files.

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