Basis tab

The Basis tab provides access to basis set information, including the plane-wave energy cutoff, FFT grid specification, and finite basis set correction.

Use custom energy cutoff: When selected, you can specify a custom value for the energy cutoff in the adjacent text box. Otherwise the text box displays the energy cutoff appropriate for the Energy cutoff quality parameter on selected on the Electronic tab.

Density: Specify the quality of the FFT grid. Three options are available:

Standard
Fine
Precise

The FFT grid parameters are chosen such that they provide a good representation of the electron density. The Precise level provides an exact representation of the density for the given energy-cutoff. The Standard and Fine levels use a reduced FFT grid to speed up the calculation. Some aliasing occurs as a result, but the effect on the SCF solution is usually small. The FFT grid parameters used in the CASTEP calculation display as derived, using the current energy-cutoff and unit cell parameters.

Augmentation density scaling factor: Specify a finer grid for representation of the augmentation charge density in the atomic core region. By default, CASTEP uses the same grid as for the standard electron density. It is reasonable in some cases to increase the augmentation grid by a factor of up to 4, to improve the accuracy of calculations. You can control this with the Quality option on the Setup tab of the CASTEP Calculation dialog.

Apply finite basis set correction: Specify when to use a finite basis set correction in the evaluation of energy and stress. You can control this with the Quality option on the Setup tab of the CASTEP Calculation dialog.

This correction gives a volume dependent addition to the energy that may result in a significant correction to any calculated stress. It is particularly important where you require an accurate stress or cell geometry optimization. Three options are available:

Smart - apply the correction only when stress is required. This includes any cell geometry optimization.
Always - apply the correction to all calculations.
Never - do not apply a correction, even when calculating stress. Any result for stress or cell geometry may be unreliable.

Correction mode: Specify how to calculate the finite basis set correction. The evaluation of the correction requires an estimate for the derivative of the total energy with respect to the logarithm of the plane-wave energy cutoff. The options are:

Automatic - automatically calculates the energy derivative using numerical differentiation.
Manual - allows you to manually specify the energy derivative.

Numerical differentiation using: Specify the number of energy points to use in evaluating the energy derivative required for the finite basis set correction. This evaluates a converged SCF energy at each point.

Available only if you select the Automatic Correction mode.

Energy derivative w.r.t. cutoff: Specify the value of the energy derivative required for the finite basis set correction.

Available only if you select the Manual Correction mode.

Access methods

Menu	Modules \| CASTEP \| Calculation \| Electronic \| More... \| Basis
Toolbar	\| Calculation \| Electronic \| More... \| Basis

Basis tab

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