CASTEP > Tasks in CASTEP > Setting up CASTEP calculations > Requesting electronic, structural, and vibrational properties > Requesting electronic excitations

CASTEP allows you to calculate electronic excitation energies and the corresponding oscillator strengths (transition probabilities) using time-dependent density functional theory (TD-DFT). These excitation energies correspond to the locations of absorption peaks in the optical spectrum of a material and represent an improvement over Kohn-Sham excitation energies in terms of accuracy. The calculations are carried out using the Tamm-Dankoff approximation (Hirata and Head-Gordon 1999). Knowledge of the excitation energies and corresponding transition probabilities allows one to calculate full set of optical properties.

To calculate TD-DFT optical properties

- Choose Modules | CASTEP | Calculation from the menu bar.
- Select the Properties tab.
- Check the Electronic excitations option in the properties list.
- Check the Calculate energies checkbox to obtain excitation energies.
- Check the Calculate optical properties checkbox to calculate the transition probabilities which can be used for analysis.
- If necessary, modify the number of excitations which should be calculated.

The excitation data will be reported in the `<seedname>_TDDFT.castep`

output file.

In certain applications the lifetime of an excited state can be sufficiently long to justify the need for geometry optimization of this state; this requires calculation of forces within TD-DFT formalism which is provided by CASTEP.

To optimize geometry for a particular excitation

- Choose Modules | CASTEP | Calculation from the menu bar.
- Select the Properties tab.
- Check the Electronic excitations option in the properties list.
- Check the Optimize geometry checkbox.
- Modify the excitation number which should have its geometry optimized.

The optimized structure for the specified excitation will be saved in the `<seedname>_TDDFT_GO.xsd`

output file.

The current version of CASTEP has a number of limitations related to the TD-DFT calculation of electronic excitations:

- The input structure should be a molecule in a periodic box
- The structure must be an insulator (the Metal checkbox on the Setup tab of the CASTEP Calculation dialog must be unchecked)
- Norm-conserving pseudopotentials must be used (see the Setting up pseudopotentials topic)
- Fixed orbital occupancies are required (see the Setting up SCF parameters topic)
- Only a Gamma point calculation is allowed, which should be sufficiently accurate for a "molecule in a box" geometry
- LDA+U calculations are not supported

CASTEP Calculation dialog

Electronic excitations selection

Accelrys Materials Studio 8.0 Help: Wednesday, December 17, 2014