Requesting electronic excitations (TD-DFT)

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

1. Choose Modules | CASTEP | Calculation from the menu bar.
2. Select the Properties tab.
3. Check the Electronic excitations (TD-DFT) option in the properties list.
4. Check the Calculate energies checkbox to obtain excitation energies.
5. Check the Calculate optical properties checkbox to calculate the transition probabilities which can be used for analysis.
6. 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

1. Choose Modules | CASTEP | Calculation from the menu bar.
2. Select the Properties tab.
3. Check the Electronic excitations (TD-DFT) option in the properties list.
4. Check the Optimize geometry checkbox.
5. 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.

Limitations of electronic excitation calculations in CASTEP

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

• 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

See Also:

CASTEP Calculation dialog
Electronic excitations (TD-DFT) selection