CASTEP > Dialogs in CASTEP > CASTEP Calculation dialog > Electronic tab > CASTEP Electronic Options dialog > 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 checked, a custom value for the energy cutoff can be specified 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. At the Precise level, an exact representation of the density is provided for the given energy-cutoff. At the Standard and Fine levels, a reduced FFT grid is used to speed up the calculation. Some aliasing will occur as a result but the effect on the SCF solution is usually small. The FFT grid parameters that will be used in the CASTEP calculation are displayed 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 the same grid is used as for the standard electron density; it is reasonable in some cases to increase the augmentation grid by a factor of up to 4 in order to improve the accuracy of calculations. This can be controlled by the Quality setting 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. This can be controlled by the Quality setting 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 an accurate stress or cell geometry optimization is required. Three settings are available:

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

Correction mode: Specify how the finite basis set correction should be calculated. 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 - the energy derivative will be calculated automatically using numerical differentiation.
- Manual - the energy derivative is specified manually.

Numerical differentiation using: Specify the number of energy points to use in evaluating the energy derivative required for the finite basis set correction. A converged SCF energy will be evaluated at each point.

This option is enabled only if the Automatic Correction mode is selected.

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

This option is enabled only if the Manual Correction mode is selected.

Access methods

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

Toolbar | | Calculation | Electronic | More... | Basis |

Plane wave basis set

Setting electronic options

Electronic tab - CASTEP Calculation dialog

SCF tab - CASTEP Electronic Options dialog

k-points tab - CASTEP Electronic Options dialog

Potentials tab - CASTEP Electronic Options dialog

Accelrys Materials Studio 8.0 Help: Wednesday, December 17, 2014