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
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
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:
CASTEP Calculation dialog
Electronic excitations selection