Requesting optical properties (DFT)

CASTEP currently implements two approaches for optical properties calculations; one is based on standard DFT Kohn-Sham orbitals and the other uses more accurate but much more time-consuming time-dependent DFT (TD-DFT) theory. In this section we consider only DFT Optical properties. Setting up TD-DFT optical properties calculations are described in the section Requesting electronic excitations (TD-DFT).

Calculating optical properties produces electronic energies on the Monkhorst-Pack mesh of k-points and the matrix elements for electronic interband transitions. The CASTEP Analysis dialog can be used to generate observable optical properties based on this information. Detailed analysis of the impact of calculation parameters (k-point set, number of empty bands, and so on) on the accuracy of the optical properties generated can be found in the Optical properties theory topic.

You are not required to use the same exchange-correlation functional in the optical properties calculation as in the main calculation. This allows you to select a nonlocal screened-exchange functional to study the optical properties after performing, for example, geometry optimization with a variable cell using the GGA functional.

To calculate optical properties

Choose Modules | CASTEP | Calculation from the Materials Studio menu bar.
Select the Properties tab.
Check the Optical properties option on the properties list.
If necessary, modify the Number of empty bands to include more or less states in the conduction band.
The number of empty bands used should be greater when calculating optical properties than for band structure or density of states calculations (see the Optical properties theory topic for more details).
Optionally, adjust the density of Monkhorst-Pack k-points used for the optical properties calculation by using k-point set dropdown list.
Click the More... button to open the CASTEP Optical Properties Options dialog. This dialog gives you complete control over setting up the grid.
Check Use separate XC functional for optics calculation on the CASTEP Optical Properties Options dialog if you want to use, for example, a nonlocal functional for the optical properties calculation.
Select the alternative Functional you wish to use from the dropdown list on the same dialog.

Requesting optical properties (DFT)

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