The CASTEP Density of States Options dialog allows you to specify the k-point set and the eigenvalue convergence criterion used for density of states 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 DOS calculation: When checked, indicates that a different exchange-correlation functional will be used for the density of states 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 density of states 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 density of states 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 DOS calculation checkbox is checked.
Band energy tolerance: Specify the convergence criterion, in eV, for electronic eigenvalues during the density of states 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 | Density of states | More...|
|Toolbar||| Calculation | Properties | Density of states | More...|
Properties tab - CASTEP Calculation dialog