The CASTEP Phonon Properties Setup dialog allows you to set additional parameters affecting a phonon calculation.
Method: Select the technique to be used to calculate phonon frequencies from the dropdown list. Available options are:
The finite displacement technique can be used for metals, ultrasoft potentials, spin-polarized systems, etc. and allows you to obtain phonon frequencies with no limitations on the settings or the physical properties of the system.
If LO-TO splitting is requested (i.e., by checking the Calculate LO-TO splitting checkbox on the Properties tab of the CASTEP Calculation dialog when Phonons is selected), then the restrictions on the system type and settings (insulator, norm-conserving potentials, non-spin-polarized) are the same for both the linear response and finite displacement techniques.
Use interpolation: When checked, indicates that the real space dynamical matrix will be calculated using the Monkhorst-Pack mesh defined by the separation parameter.
The Use interpolation option is enabled only if Linear response is selected from the Method dropdown list.
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.
The q-vector grid spacing for interpolation control is enabled only if Linear response is selected from the Method dropdown list and the Use interpolation checkbox is checked.
Convergence tolerance: Specify the convergence criterion, in eV Å-2, for the force constants during a phonon properties run.
The Convergence tolerance control is enabled only if Linear response is selected from the Method dropdown list.
Supercell defined by cutoff radius: Specify the real space cutoff radius, in Å, for dynamical matrix calculations. This value will be used 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, it is confirmed that the original cell is large enough. For non-3D periodic systems a reminder that a supercell cannot be constructed is displayed.
The Supercell defined by cutoff radius control is enabled only if Finite displacement is selected from the Method dropdown list.
Quality: Set the quality of the q-vector set 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)|
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.
Quality: Set the quality of the q-vector set 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)|
a single q-vector at (0,0,0) will be used
For Linear response calculations, if Gamma point is selected and the Use interpolation checkbox is checked frequencies will be calculated using 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. Such calculations can be justified only as a way of testing the accuracy of interpolation scheme (directly calculated frequencies are more accurate). To run calculations only at the Gamma point you should uncheck the Use interpolation checkbox.
More...: Provides access to the CASTEP Phonon Density of States Options dialog, which allows you to specify the q-vector set.
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