# Optic

This page gives hints on how to compute linear and non-linear optical properties in the independent-particle approximation with the Abinit package.

## Introduction¶

Optical and non-linear optical properties can be computed with different levels of approximation.

The simplest (and fastest) approach relies on the independent-particle approximation (IPA): the electrons are supposed independent of each other when reacting to the optical perturbation (even if the initial computation of the band structure includes interactions in a mean-field sense, like with DFT). This approximation is also referred to as a “Sum-Over-States” approach (SOS). This neglects all electron-hole interaction (so no excitonic effects), but might provide meaningful results in many case, sometimes even quantitatively. A first problem is linked with the erroneous band gap of the material, but this can be corrected by a scissor approximation, see scissor@optic.

In Abinit one can either work in the IPA (see below), or take into account the excitonic effects, see topic_BSE.

In the Abinit package, there are two different utilities to compute optical responses in the independent-particle approximation: optic help file and conducti [Mazevet2010]. They have been developed independently of each other, and thus overlap significantly. The first one computes the linear and non- linear optical properties as a function of the frequency. It provides the optical dielectric tensor, the second-harmonic generation (SHG) as well as the optical rectification tensor (or electro-optic tensor) - without the contribution from the nuclear displacements. For the further inclusion of the contribution from nuclear displacements, see topic_nonlinear.

The second utility “conducti” has more capabilities but only at the linear level, providing the electronic conductivity, dielectric tensor, index of refraction, reflectivity, absorption, the thermal conductivity, and the thermopower (electron transport, high temperature, Kubo-Greenwood formalism), the real as well as imaginary parts.

basic:

v4:

v67mbpt:

v7:

## Tutorials¶

• See The tutorial on Optic, the utility that allows to obtain the frequency dependent linear optical dielectric function and the frequency dependent second order nonlinear optical susceptibility, in the simple “Sum-Over-States” approximation.