CASTEP Phonon Properties Setup dialog
The CASTEP Phonon Properties Setup dialog allows you to specify additional parameters affecting a phonon calculation.
Method: Select the technique to use to calculate phonon frequencies. Available options are:
- Linear response
- Finite displacement
With ultrasoft potentials, you can only use the finite displacement technique.
If you request LO-TO splitting, then the restrictions on the system type and settings (insulator, norm-conserving potentials) are the same for both the linear response and finite displacement techniques. Select Calculate LO-TO splitting on the Properties tab of the CASTEP Calculation dialog when Phonons to request LO-TO splitting.
Linear response
Use interpolation: When selected, calculates the real space dynamical matrix using the Monkhorst-Pack mesh defined by the separation parameter.
q-vector grid spacing for interpolation: Specify the q-point separation parameter, in Å-1, which represents the average distance between Monkhorst-Pack mesh q-points used in the real space dynamical matrix calculations. A smaller separation parameter produces a denser mesh, giving more accurate results at the expense of longer calculation times.
Adjust phonon q-vector grid: When selected, adjusts the phonon Monkhorst-Pack mesh to always connect points on the electronic k-vector grid. This can significantly improve convergence especially for metallic cases.
You can only specify the q-vector grid spacing for interpolation and Adjust phonon q-vector grid if you select Use interpolation.
Convergence tolerance: Specify the convergence criterion, in eV Å-2, for the force constants during a phonon properties run.
You can only specify the Convergence tolerance if you select Linear response as the Method.
Finite displacement
Use: Select the method to calculate the dynamic matrix using finite displacement. Available options are:
- Several small supercells - uses the Monkhorst-Pack mesh defined by the separation parameter. When you select Finite displacement as the Method, this invokes the nondiagonal supercell approach of Lloyd-Williams and Monserrat (2015).
- One large supercell
q-vector grid spacing for interpolation: Specify the q-point separation parameter, in Å-1. This represents the average distance between Monkhorst-Pack mesh q-points used in the real space dynamic matrix calculations. A smaller separation parameter produces a denser mesh, giving more accurate results at the expense of longer calculation times.
You can only specify the q-vector grid spacing for interpolation if you select Several small supercells.
Supercell defined by cutoff radius: Specify the real space cutoff radius, in Å, for dynamical matrix calculations. CASTEP uses this value to construct an appropriate supercell. A larger cutoff radius produces more accurate results at the expense of longer calculation times.
If the cutoff radius specified creates a large supercell, its size is reported as a factor of the size of the current cell, indicating that the calculation may be computationally expensive. If the cutoff radius is reasonable, the report confirms that the original cell is large enough. For non-3D periodic systems, you receive a reminder that Materials Studio cannot construct a supercell.
You can only specify the Supercell defined by cutoff radius if you select One large supercell.
Dispersion
Quality: Specify the quality of the q-vector used for the phonon dispersion calculation. The quality level affects the approximate separation between consecutive q-vectors on the reciprocal space path. Available options and the corresponding q-vector separations are:
| Quality | q-vector separation (Å-1) |
|---|---|
| Coarse | 0.04 |
| Medium | 0.025 |
| Fine | 0.015 |
Separation: Specify the approximate separation between q-vectors in Å-1. The default depends on the selected Quality of the q-vector.
Path...: Provides access to the Brillouin Zone Path dialog, which allows you to specify a custom reciprocal space path.
Density of states
Quality: Specify the quality of the q-vector to use for the phonon density of states calculation. The quality level affects the separation between neighboring q-vectors in the Monkhorst-Pack grid. Available options and the corresponding q-vector separations are:
| Quality | k-point separation (Å-1) |
|---|---|
| Gamma |
uses a single q-vector at (0,0,0) |
| Coarse | 0.07 |
| Medium | 0.05 |
| Fine | 0.04 |
For Linear response calculations, if you select Gamma point and select the Use interpolation, this calculates the frequencies using the interpolation method. This involves calculations of phonon frequencies at multiple q-vectors in the Brillouin zone and is therefore an expensive way of generating Gamma point phonons. The only justification for such calculations is as a way of testing the accuracy of the interpolation scheme (directly calculated frequencies are more accurate). To run calculations only at the Gamma point, clear selection of Use interpolation.
More...: Provides access to the CASTEP Phonon Density of States Options dialog, where you can specify the q-vector to use.
Help: Displays
Access methods
| Menu | Modules | CASTEP | Calculation | Properties | Phonons | More... |
| Toolbar |
 | Calculation | Properties | Phonons | More...
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