Potentials tab

The Potentials tab allows you to control which pseudopotentials to use for each element in the structure.

Scheme: Select the type of pseudopotential to use. Two different types of pseudopotentials are available, ultrasoft and norm-conserving. Both of these are available as either generated on the fly (OTFG) potentials or as tabulated potentials. OTFG potentials generally provide better accuracy and consistency. You can only perform NMR calculations with OTFG potentials.

OTFG ultrasoft (default) - the preferred option, ultrasoft pseudopotentials allow you to perform 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 set of 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. Norm-conserving potentials are required for linear response-based calculations of phonon properties and of polarizabilities and for calculations with nonlocal exchange-correlation functionals (for example, screened exchange).
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.

The latest set of OTFG settings for ultrasoft pseudopotentials was developed to minimize the error with respect to fully converged all-electron DFT calculations. The error achieved by this set is 0.4 meV/atom, which puts CASTEP among the most accurate pseudopotential codes available. A full definition of the test framework and the meaning of the error is given by Lejaeghere et al. (2014), and on the website of the Delta project. The files that correspond to this set have the _2017R2 suffix. This is the recommended set of OTFG ultrasoft potentials. Use files with the _2017R2ncp suffix when you require norm-conserving potentials. This set has a 1.1 meV/atom error in the Delta project tests. In addition, provides a set of "high throughput" ultrasoft OTFG settings for scripting. This QC5 set generally requires lower cutoff energy and offers faster but less accurate calculations (1.9 meV/atom error in the Delta project tests).

Do not use the PW91 exchange-correlation functional when requesting on the fly generation of pseudopotentials. PBE, RPBE, WC, BLYP, or PBESOL are better options when you require a GGA functional.

When you specify the type of pseudopotential to use in this way, the default pseudopotential of that type for each element will be used in the calculation.

Representation: Choose between Reciprocal space and Real space representations of the pseudopotentials.

The real space representation may give a performance advantage for large systems.

More...: Provides access to the CASTEP Real Space Potentials dialog, which allows you to specify details of the transformation of pseudopotentials to a real space representation.

This is enabled only if you select Real space from the Representation list.

Selected potentials: Displays information about the selected pseudopotentials for each element in a grid.

There is a row in the grid for each element displaying the element type, current pseudopotential, energy, and valence (Z). To select a different pseudopotential for a particular element, click in the appropriate pseudopotential grid cell and choose a new pseudopotential from the list.

If you change the default pseudopotentials, the Scheme displays as Custom.

Show potentials for all elements: When selected, the Selected potentials list displays information about all elements in the periodic table for which pseudopotentials are available. Otherwise, only elements present in the current structure are displayed in the list. Default = unchecked.

View: Displays a modal view on the selected pseudopotential file.

Electric field: Specify the external electric field strength, in eV/Å/e.

Field direction (X Y Z): Vector specifying the direction of the electric field.

Only systems that contain molecules or slabs are appropriate for electric field calculations.
For slabs, the field must be along the surface normal. In both cases, you must position atoms in the middle of the cell and the symmetry must be P1.

The field direction is specified in Cartesian coordinates. The values of the X, Y, and Z direction components of the electric field are interrelated. The three field components define a unit vector. When you start a job, the direction components are normalized to give a unit vector, which is then multiplied by the field magnitude to give the actual field components. Entering a vector with all zero components has the same effect as entering a (111) vector.

Access methods

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Potentials tab

External Potential

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