CASTEP > Dialogs in CASTEP > CASTEP Calculation dialog > Setup tab - CASTEP Calculation dialog

Setup tab

The Setup tab allows you to choose the type and quality of calculation that CASTEP will perform along with other basic input options such as exchange-correlation functional, spin polarization, and total charge.

Task: Select the type of calculation that CASTEP will perform from the dropdown list. Available options are:

More...: Provides access to further options for the selected task: opening the CASTEP Geometry Optimization, CASTEP Dynamics, CASTEP Elastic Constants, CASTEP Transition State Search , or CASTEP TS Confirmation dialogs.

Quality: Set the overall quality of a single-point energy, geometry optimization, or dynamics calculation. This quality affects the basis set, k-point, and SCF convergence criteria, along with the convergence criteria for the Geometry Optimization, Elastic Constants, and TS Search tasks. Available options are:

Use the Coarse quality setting for a quick assessment of the calculation. Use the Express quality setting to achieve a good compromise between speed and accuracy which is appropriate for most calculations. Use Fine and Ultra-fine quality settings for calculations that require very high accuracy, at the expense of longer calculation times. Carry out convergence testing for highly sensitive calculations by increasing the quality setting and possibly by increasing accuracy of the basis set and the k-point sampling.

The Quality setting affects all relevant task parameters that control the precision of the simulation. If any parameter is set to a value different from that specified by the overall quality level, the Quality is displayed as Customized.

Functional: Select the type of DFT exchange-correlation potential to be used in the calculation. Choose the class of functional from the first dropdown list, then select the specific functional from the second dropdown list.

Nonlocal potentials:

Nonlocal exchange can only be used for insulators using All Bands/EDFT Electronic minimizer for Energy and Geometry Optimization (without cell optimization) tasks. It is not compatible with stress, NMR, phonon, or polarizability calculations. The formalism cannot be used with mixture atoms and only norm conserving potentials can be used. Note that stress calculations, and hence cell optimization and the Elastic Constants task, are supported for screened exchange versions, sX and sX-LDA.

There is an important difference between standard DFT calculations with local exchange-correlation potentials and the nonlocal exchange case. The potential used in the latter scenario depends on the wavefunctions at SCF k-points, while in the former case, the potential depends only on electron density. This difference can make 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.

Use method for DFT-D correction: When checked, the selected method will be used for dispersion corrections. Available options are:

The option selected will automatically update the Use custom DFT-D parameters setting on the DFT-D tab of the CASTEP Electronic Options dialog.

Spin polarized: When checked, indicates that the calculation will be performed using different wavefunctions for different spins. This is known as a 'spin-polarized' or 'spin-unrestricted' calculation. If unchecked, the calculation uses the same orbitals for alpha and beta spins. This is known as a 'non-spin-polarized' or 'spin-restricted' calculation. Default = unchecked.

Use formal spin as initial: When checked, indicates that the initial value for the number of unpaired electrons for each atom will be taken from the formal spin specified for each atom. This starting value will be subsequently optimized during the calculation. Default = checked.

This option is enabled only if the Spin polarized checkbox is checked.

Atom-resolved initial spin is used only if either the Electronic minimizer control is set to Density mixing on the SCF tab of the CASTEP Electronic Options dialog or the LDA+U calculation (see below) is chosen.

Use LDA+U: When checked, indicates that LDA+U formalism is to be used. Actual values of U can be set up through the Hubbard U tab of the Electronic Configuration dialog.

This option is only enabled if the Spin polarized checkbox is checked.

The LDA+U formalism is not compatible with NMR, polarizability, or Elastic Constant calculations. The formalism cannot be used with mixture atoms or real-space pseudopotentials.

Initial spin: Specify the initial value for the number of unpaired electrons in a spin-polarized calculation. This starting value will be subsequently optimized during the calculation.

This option is enabled only if the Spin polarized checkbox is checked and the Use formal spin as initial checkbox is unchecked.

Metal: When checked this indicates that the system is metallic so that empty bands can be created and edited. When unchecked the Fix occupancy checkbox on the SCF tab of the CASTEP Electronic Options dialog is checked. When unchecked the k-point separations used by default are coarser. Default = checked.

In some calculations is not sufficient to use fixed orbital occupancies if the system under investigation should actually have a band gap according to DFT results. This applies to calculations that use the linear response formalism for vibrational properties and to calculations of electric field responses (polarizability and IR and Raman spectra). Attempting to study a metallic systems using fixed orbital occupancies will result in poor convergence and usually in unphysical results for IR and Raman intensities and for polarizability. If this is the case, check whether your system is metallic by running either density of states or band structure calculations.

Charge: Specify the total charge on the unit cell.

Access methods

Menu Modules | CASTEP | Calculation | Setup
Toolbar | Calculation | Setup
See Also:

CASTEP
CASTEP Calculation dialog
CASTEP Geometry Optimization dialog
CASTEP Dynamics dialog
CASTEP Elastic Constants dialog
CASTEP Transition State Search dialog
CASTEP TS Confirmation dialog

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