# optic input variables¶

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.

Mentioned in topic(s): topic_Optic
Variable type: real
Dimensions: scalar
Default value: 1.d-3 Ha

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter applies a broadening to the spectrum and is used to avoid divergences in the sum-over-states approach. The sum-over-states approach to the linear and nonlinear susceptibilities inevitably include resonant denominators of the form $(\omega - \omega_{nm})^{-1}$, see for example Eq. 46 of [Sharma2004]. Numerically these denominators lead to infinities. In order to avoid them one could do one of two things. One could change the sum over k-points to integration, and then use the linear tetrahedron method (see [Hughes1996] for details). Another way to get around the problem, as we will do in the present case, is to avoid the singularities by adding a small imaginary contribution to the denominator. This addition 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.

## ddkfile¶

Mnemonics: DDK FILE
Mentioned in topic(s): topic_Optic
Variable type: string
Dimensions: scalar
Default value: None
Comment: no default

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the name of the file containing the matrix elements of the $d/dk$ operator in direction X, as the string ddkfile_X. This file should have been produced by a preparatory Abinit run. This file must not contain the first-order wavefunctions, and may be generated using prtwf 3. Make sure that the number of bands, spin channels, and k-points are the same in all the files.

## domega¶

Mnemonics: Delta OMEGA
Mentioned in topic(s): topic_Optic
Variable type: real
Dimensions: scalar
Default value: 1.d-3 Ha

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the step size $\Delta\omega$ for the grid over which the optic utility computes the susceptibilities. The maximum energy is set by the companion parameter maxomega. The susceptibilities are thus computed at maxomega/domega energy points (zero excluded). In order to capture more features, decrease the step size to get a finer energy grid. In order to go to higher frequency, increase the maximum.

## lin_comp¶

Mnemonics: LINear COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: ([‘num_lin_comp’])
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the directions of the num_lin_comp requested components of the dielectric tensor. The components are specified in cartesian coordinates, where 1, 2, and 3 represent x, y, and z respectively. For example, 11 represents the xx component, and 32 represents zy. There should be num_lin_comp entries. Note that these directions are denoted by $a$ and $b$ in [Sharma2004].

## linel_comp¶

Mnemonics: LINear ELectro-optic COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: ([‘num_linel_comp’])
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the directions of the num_linel_comp requested components of the linear electro-optical susceptibility. The components are specified in cartesian coordinates, where 1, 2, and 3 represent x, y, and z respectively. For example, 111 represents the xxx component, and 321 represents zyx. There should be num_linel_comp entries.

## maxomega¶

Mnemonics: MAXimum value of OMEGA
Mentioned in topic(s): topic_Optic
Variable type: real
Dimensions: scalar
Default value: 1 Ha

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the maximum energy for the grid over which the optic utility computes the susceptibilities. The grid step size is set by the companion parameter domega. The susceptibilities are thus computed at maxomega/domega energy points (zero excluded). In order to capture more features, decrease the step size to get a finer energy grid. In order to go to higher frequency, increase the maximum.

## nonlin_comp¶

Mnemonics: NON-LINear COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: ([‘num_nonlin_comp’])
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies the directions of the num_nonlin_comp requested components of the second-order nonlinear dielectric tensor. The components are specified in cartesian coordinates, where 1, 2, and 3 represent x, y, and z respectively. For example, 111 represents the xxx component, and 321 represents zyx. There should be num_nonlin_comp entries. Note that these directions are denoted by $a$ and $b$ in [Sharma2004].

## num_lin_comp¶

Mnemonics: NUMber of LINear COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: scalar
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies how many components (out of 9 possible) of the linear optical dielectric tensor to calculate. Some of these may be either equal to each other, or zero, depending upon the symmetry of the material (for details see [Draxl2006]). The directions of the requested components are specified by the parameter lin_comp.

## num_linel_comp¶

Mnemonics: NUMber of LINear ELetro-optic COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: scalar
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies how many components (out of 27 possible) of the linear electro-optical susceptibility to calculate. Some of these may be either equal to each other, or zero, depending upon the symmetry of the material. The directions of the requested components are specified by the parameter linel_comp.

## num_nonlin_comp¶

Mnemonics: NUMber of NON-LINear COMPonents
Mentioned in topic(s): topic_Optic
Variable type: integer
Dimensions: scalar
Default value: 0

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter specifies how many components (out of 27 possible) of the second-order nonlinear optical tensor to calculate. Some of these may be either equal to each other, or zero, depending upon the symmetry of the material. The directions of the requested components are specified by the parameter nonlin_comp.

## scissor¶

Mnemonics: SCISSOR operator
Mentioned in topic(s): topic_Optic
Variable type: real
Dimensions: scalar
Default value: 0.0
Comment: in Ha

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

This parameter provides a fixed shift to all the conduction bands. As LDA/GGA are known to underestimate the band-gap by a significant amount in some cases, in order to obtain a reasonable optical spectrum and make a realistic comparison with experiments one needs to correct for this. The scissors shift is normally chosen to be the difference between the experimental and theoretical band-gap, and simply shifts the conduction bands. Alternatively, one may determine the self energy using the GW approach, in which case the opening of the gap due to the GW correction can be used as the scissor shift.

## tolerance¶

Mnemonics: TOLERANCE
Mentioned in topic(s): topic_Optic
Variable type: real
Dimensions: scalar
Default value: 1.d-3 Ha

Test list (click to open). Very frequently used, [13/13] in all optic tests, [3/3] in optic tutorials

## wfkfile¶

Mnemonics: WaveFunction K FILE
Mentioned in topic(s): topic_Optic
Variable type: string
Dimensions: scalar
Default value: None
Comment: no default