The CASTEP Optical Properties Options dialog allows you to specify the density of the k-point set and the eigenvalue convergence criterion used for optical properties calculations.
The Monkhorst-Pack k-point grid to be used in the calculation can be specified in one of four ways:
Gamma point only: When selected, indicates that a single k-point at (0,0,0) will be used for the density of states calculation.
Quality: When selected, indicates that the k-point grid will be generated using a k-point separation appropriate to the specified quality level. Select the desired quality level from the dropdown list. Available options are:
The k-point separations associated with the three Quality settings are as follows:
|Quality||k-point separation (Å-1)|
Separation: When selected, indicates that the k-point grid will be generated according to the specified k-point separation. Specify the k-point separation, in Å-1, in the associated text box.
When the Separation option is selected, the Monkhorst-Pack parameters are derived to give the specified separation between neighboring grid points.
Custom grid parameters: When selected, indicates that the k-point grid will be generated using the Monkhorst-Pack grid parameters and the origin shift in fractional reciprocal space coordinates specified in the Grid parameters and Origin shift text boxes, respectively.
Grid parameters: Specify the Monkhorst-Pack grid parameters in each of the lattice directions.
Actual spacing: Displays the k-point separation, in Å-1, resulting from the currently specified Monkhorst-Pack grid parameters in each of the lattice directions.
Origin shift: Specify the offset of the Monkhorst-Pack grid in fractional reciprocal space coordinates.
The Grid parameters and Origin shift controls are enabled only if the Custom grid parameters option is selected.
Display points...: Displays the number and fractional coordinates of the reciprocal space mesh points that would be generated using the currently specified parameters.
The actual set of k-points that will be used in the calculations may be altered if the symmetry of the system changes.
Use separate XC functional for optics calculation: When checked, indicates that a different exchange-correlation functional will be used for the optical properties calculation from the one used in the self-consistent calculation.
This feature is particularly useful for carrying out expensive, screened exchange calculations. Perform the main run with the LDA or GGA functional, then invoke screened or exact exchange in non self-consistent mode during the band structure calculation.
There is an important difference between standard DFT calculations with local exchange-correlation potentials and the non-local exchange case. The potential used in the latter scenario depends on the SCF k-points, while in the former case, the potential depends only on electron density. This difference can make all properties calculations a lot more expensive in terms of memory usage and CPU time, so it is advisable to limit the number of SCF k-points that you use in such situations. This is particularly relevant for small unit cells, where the default settings may generate a very large k-point set.
Functional: Select the type of DFT exchange-correlation potential to be used in the optical properties calculation.
Choose the type of functional from the first dropdown list. Available options are:
Then, if appropriate, select the specific functional from the second dropdown list. Depending on the functional type selected from the first dropdown list, some of the following options may be available:
The Functional control is enabled only if the Use separate XC functional for optics calculation checkbox is checked.
Band energy tolerance: Specify the convergence criterion, in eV, for electronic eigenvalues during the optical properties calculation. The calculation will stop when all electronic eigenvalues change by less than this value over a single iteration of the electronic structure.
|Menu||Modules | CASTEP | Calculation | Properties | Optical properties | More...|
|Toolbar||| Calculation | Properties | Optical properties | More...|
Properties tab - CASTEP Calculation dialog