# Time-Dependent Density Functional Theory for neutral excitation energies

### This page gives hints on how to perform time-dependent density-functional theory calculations of neutral excitation energies with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (FJ)

Mentioned in help_features#7.

### Table of content:

- 1. Introduction.
- 2. Related lesson(s) of the tutorial.
- 3. Related input variables.
- 4. Selected input files.

### 1. Introduction.

For finite systems (atoms and molecules), excited states can be computed within TDDFT (Casida approach - only norm-conserving pseudopotentials). See the explanations given in the tddft lesson of tutorial. The iscf input variable must be set to -1.

In the non-spin-polarized case, spin-singlet as well as spin-triplet excitations are computed. Spin-polarized case is also available.
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### 2. Related lesson(s) of the tutorial.

- The lesson on TDDFT deals with the computation of the excitation spectrum of finite systems, thanks to the Time-Dependent Density Functional Theory approach, in the Cassida formalism.
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### 3. Related input variables.

Compulsory input variables:

... iscf [Integer for Self-Consistent-Field cycles]

Basic input variables:

... boxcenter [BOX CENTER]

Useful input variables:

... ixc [Index of eXchange-Correlation functional]
... td_maxene [Time-Dependent dft : MAXimal kohn-sham ENErgy difference]
... td_mexcit [Time-Dependent dft : Maximal number of EXCITations]
... %xclevel [eXchange Correlation functional LEVEL]
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### 4. Selected input files.

WARNING : as of ABINITv8.6.x, the list of input files provided in the specific section of the topics Web pages is still to be reviewed/tuned. In some cases, it will be adequate, and in other cases, it might be incomplete, or perhaps even useless.
The user can find some related example input files in the ABINIT package in the directory /tests, or on the Web:

tests/v1/Input: t69.in t70.in

tests/v2/Input: t42.in

tests/v3/Input: t55.in

tests/v5/Input: t61.in t62.in

tests/paral/Input: t05.in
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