Electronic tab

The Electronic tab allows you to specify the parameters associated with the electronic Hamiltonian. These include the basis set, pseudopotentials, SCF convergence criteria, and k-point set.

Energy cutoff: Select the precision used for the plane wave basis set.

• Coarse
• Medium
• Fine
• Ultra-fine

The actual value of the energy cutoff corresponding to the selected quality for the current structure also displays.

Calculation of the NMR parameters may require a higher Energy cutoff than for total energy or geometry optimization calculations. Study the convergence of the results as you increase the Energy cutoff value. Generally, a cutoff between 270 and 400 eV produces convergence of the shielding constant to about 1 ppm. In some cases, you might need to increase the cutoff up to 550 eV.

SCF tolerance: Select the threshold used to determine whether an SCF has converged. Available options and associated convergence thresholds are:

• Coarse - 10^-5 eV/atom
• Medium - 2 × 10^-6 eV/atom
• Fine - 10^-6 eV/atom
• Ultra-fine - 5 × 10^-7 eV/atom

When the main quality is Express, the SCF tolerance is automatically Express. This corresponds to 10^-4 eV/cell.

Energy tolerances per: Specifies whether to calculate energy tolerances as the energy change per atom (Atom is the default) or per unit cell. The Cell value is enforced when the main quality, and hence the SCF tolerance quality, is Express.

k-point set: Define the number of integration points to use to integrate the wavefunction in reciprocal space. Available options are:

• Gamma - a single point at (0, 0, 0)
• Coarse
• Medium
• Fine

Pseudopotentials: Select the type of pseudopotential to use. Two different types of pseudopotentials are available, ultrasoft and norm-conserving; both of them are available as either generated on the fly (OTFG) potentials or as tabulated potentials. OTFG potentials are generally preferred for reasons of accuracy and consistency. NMR calculations required OTFG potentials.

• OTFG ultrasoft (default) - the preferred option, ultrasoft pseudopotentials allow calculations with lower energy cutoffs. On-the-fly generation produces potentials that are consistent between solid state and pseudo-atom calculations since they use the same exchange-correlation functional throughout. The OTFG ultrasoft pseudopotentials introduced in Materials Studio 8.0 provides results in close agreement with all electron calculations and is very accurate for describing ground state structures.
• OTFG norm conserving - typically harder and therefore more computationally expensive than ultrasoft pseudopotentials. Linear response-based calculations of phonon properties and of polarizabilities and calculations with nonlocal exchange-correlation functionals (for example, screened exchange) require norm-conserving potentials.
• Ultrasoft - tabulated ultrasoft pseudopotentials, provided for compatibility with earlier versions of Materials Studio.
• Norm conserving - tabulated norm-conserving pseudopotentials, provided for compatibility with earlier versions of Materials Studio.

Use the Electronic Options dialog to exercise more control over which pseudopotentials to use for each element.

Relativistic treatment: The way OTFG pseudopotentials incorporate relativistic effects. Options are:

• Schroedinger
• Koelling-Harmon (default)
• ZORA
• Dirac

Use the Dirac solver when requesting spin-orbit coupling and on the fly generated pseudopotentials.

Use core hole: Specify whether to include core holes in the calculation. You can use core holes in core level spectroscopy calculations. Define core holes using the Core Hole tab of the Electronic Configuration dialog.

For calculations exploiting core holes, you must use on-the-fly pseudopotentials.
If you do not select the Use core hole checkbox, the calculation does not preserve any core holes in the input system.

CASTEP calculations with core holes assume by default that an electron is removed from the system, so it is treated as having a charge of 1 (or one more than specified in the Charge field on the Setup tab of the CASTEP Calculation dialog). In some circumstances, for example when studying impurities, it might be more appropriate to consider the system as charge neutral. In order to achieve this you should specify the Charge of -1 on the Setup tab, in this way the CASTEP input file will have a zero charge. It is also possible to edit the seedname.param file to specify a fractional charge. The strength of the core hole can be adjusted by editing the seedname.cell file and modifying the pseudopotential definition string (for example, change {1s1} to {1s0.5} to use a core hole with the charge of 0.5).

Use solvation model: When selected, uses the implicit solvation model. The Solvent tab of the CASTEP Electronic Options dialog allows you to define the solvent characteristics.

The implicit solvation model allows you to simulate a solvent environment only for "molecule in a box" calculations. Materials Studio automatically positions isolated molecule structures roughly in the center of a cell. Ensure that the cell is large enough to contain a significant amount of vacuum surrounding the molecule.

Use at least a Fine FFT grid Density for solvation calculations; the Precise density is even better. You can specify the FFT grid Density on the Basis tab of the CASTEP Electronic Options dialog. Also use an increased value of Augmentation density scaling factor, for example 1.5-2.0 instead of the default value of 1.0.

An increased FFT density and energy cutoff can help to eliminate convergence errors for the multigrid solver for electrostatics in a solvent.

More...: Provides access to the CASTEP Electronic Options dialog, where you can configure further details of the electronic Hamiltonian.

Access methods

Menu	Modules | CASTEP | Calculation | Electronic
Toolbar	| Calculation | Electronic

See Also:

Plane wave basis set
Pseudopotentials
Requesting core level spectroscopy
Setting up a core hole calculation
CASTEP Calculation dialog
CASTEP Electronic Options dialog