Setting Up Pseudopotentials

Ultrasoft and Norm-Conserving Pseudopotentials

CASTEP uses on-the-fly generated ultrasoft pseudopotentials (USP) by default. These are generally more accurate and more efficient than norm-conserving pseudopotentials (NCP).

USP implementation allows you to run CASTEP calculations with a lower energy cutoff than their NCP counterparts producing a clear advantage in terms of the calculation time. However, USP formalism is more complex and becomes close to intractable for such complex concepts as linear response implementation for phonons or NMR properties, or for nonlocal exchange-correlation functionals. There is a number of tasks and properties that CASTEP can address only with norm-conserving potentials.

There are two types of pseudopotentials available in Materials Studio: tabulated and generated on-the-fly.

Pseudopotentials Generated On-the-Fly

CASTEP server code allows you to generate pseudopotentials on the fly, that is, you can provide the parameters that govern the generation rather than a tabulated file from the database. This approach has a number of advantages; for example:

• It uses the same exchange-correlation functional in the atomic and solid-state calculations.

• It is possible to generate "softer" or "harder" potentials by changing the core radius.

• It is possible to study excited configurations with a core hole, and so on.

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).

It is not recommended to 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.

This scheme was validated for all elements of the Periodic Table and the files themselves include the results of the tests. You can also create customized versions of the pseudopotential generation settings. In which case, test the resulting potentials thoroughly for convergence and transferability by performing calculations for a variety of systems and at different energy cutoffs. For details of the format of the on-the-fly generation settings file, see CASTEP file formats - OTFG.

On-the-fly generation of pseudopotentials involves the solution of the electronic structure of an atom and a pseudoatom. You can perform this step either in a non-relativistic (Schroedinger) framework, using one of the available scalar relativistic approaches (Koelling-Harmon or ZORA), or using a fully relativistic atomic solver (Dirac). It is important to use a scalar relativistic treatment for improved accuracy of calculations for heavy elements.

Tabulated Pseudopotentials

Tabulated USP files in the database have a .usp or .uspcc extension, tabulated norm-conserving files have a .recpot extension.

Use different tabulated ultrasoft potentials for LDA and GGA exchange-correlation functionals. The naming convention in the database is <element name>_<ID>.usp[cc] for LDA functionals and <element name>_<ID>PBE.usp[cc] for GGA functionals. By default, Materials Studio tries to find the best fit of the potentials to the exchange-correlation potentials, although you can override this behavior.

The database of NCPs available in Materials Studio is rather old and in some cases might contain potentials that have not been tested sufficiently. An alternative is to use on-the-fly generated pseudopotentials that are more modern and more extensively tested.

Besides changing the global selection of pseudopotentials, it is possible to change the individual selections for each element according to the selected value of the exchange-correlation functional.

Individual selections are also useful for elements such as oxygen and silicon where additional soft versions of the USPs are available. These require a smaller energy cutoff but may be less transferable than the standard potentials.

Nonlinear Core Corrected Pseudopotentials

Materials Studio provides USPs with nonlinear core correction (NLCC) for some elements, most notably transition metals. These potential files have a .uspcc extension. In some cases, NLCC potentials are more accurate than the corresponding non-corrected USPs and others they provide a cheaper and less accurate alternative to the non-corrected USPs.

You can distinguish between these two cases by comparing the Valence values displayed on the Potentials tab of the CASTEP Electronic Options dialog.

For example, Mn_00.usp and Mn_00.uspcc both use seven valence electrons. This means that the NLCC version is more accurate (and it is therefore the default potential for Mn). However, Cr_00.usp uses 14 valence electrons, while Cr_00.uspcc uses only six. This means that Cr_00.usp treats semicore s and p states explicitly as valence states and is thus more accurate than the NLCC version.

To change global pseudopotential selection

1. Choose Modules | CASTEP | Calculation from the Materials Studio menu bar.
2. Select the Electronic tab.
3. Set the Pseudopotentials option to either Ultrasoft or Norm-conserving using the list.

To change pseudopotential selection for a given element

1. Choose Modules | CASTEP | Calculation from the Materials Studio menu bar.
2. Select the Electronic tab.
3. Click More... to open the CASTEP Electronic Options dialog.
4. Select the Potentials tab.

5. Set the Pseudopotential for each element from the list that appears in the appropriate grid cell when it is edited.

   For example, for Mg Materials Studio provides the following pseudopotentials: Mg_00.usp, Mg_00PBE.usp, Mg_00PW91.usp, Mg_00.recpot.

6. Click View to display the selected pseudopotential scheme.

Pseudopotential Database

Materials Studio provides the pseudopotentials as ASCII text files in the folder <MS_INSTALL>\share\Resources\Quantum\CASTEP\Potentials. The headers of these files contain useful information about:

• Recommended cutoff energies for different convergence levels.
• Pseudopotential generation data for core radii, electronic configuration, exchange-correlation potential, etc.
• Convergence testing for energy and forces on atoms for a simple system as a function of the energy cutoff.
• Scientific testing for lattice constants and molecular bond lengths compared to experiment.

This information might be useful when deciding which potential to use, or when writing up the results of a calculation. In addition, you can use the core radii as a guide to selecting a real-space transformation radius.

See Also:

Setting electronic options
Electronic tab - CASTEP Calculation dialog
Potentials tab - CASTEP Electronic Options dialog