CASTEP > Tasks in CASTEP > Setting up CASTEP calculations > Setting electronic options > Setting up pseudopotentials

Setting up pseudopotentials

Ultrasoft and norm-conserving pseudopotentials

CASTEP uses ultrasoft pseudopotentials (USP) by default. These are generally more accurate and more efficient than norm-conserving pseudopotentials (NCP). USP files in the database have a .usp or .uspcc extension, norm-conserving files have a .recpot extension.

Different ultrasoft potentials should be used 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, Material Studio tries to find the best fit of the potentials to the exchange-correlation potentials, although this behavior can be overridden.

USP implementation allows CASTEP calculations to be run 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. As a result, there is a number of tasks and properties that CASTEP can address only with norm-conserving potentials. 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 to using this database is to generate NCPs using the open source software package Opium, The Optimized Pseudopotential Interface / Unification Module. This package allows you to generate files in .recpot format using the latest pseudopotential generation technology.

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.

Pseudopotentials generated on the fly

CASTEP server code allows you to generate pseudopotentials on the fly, i.e., you can provide the parameters which govern the generation rather than a file from the database. This approach has a number of advantages; for example, the same exchange-correlation functional is used 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, etc.

The latest set of OTFG settings introduced in Materials Studio 8.0 has been developed in order to minimize the error with respect to fully converged all-electron DFT calculations. The error achieved by this set is 0.5 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 an _80 suffix.

It is not recommended to use PW91 exchange-correlation functional when requesting on the fly generation of pseudopotentials; PBE, RPBE, or PBESOL are better options when a GGA functional is required.

This scheme has been validated for all elements of the Periodic Table, the results of the tests are included in the files themselves. It is possible to create customized versions of the pseudopotential generation settings, but it is recommended to test the resulting potentials thoroughly for convergence and transferability by performing calculations for a variety of systems and at different energy cutoffs. The details of the format of the settings file for on the fly generation are provided.

CASTEP NMR calculations can be performed only by using on the fly generation scheme, since the available potentials in the database do not contain sufficient information about all-electron projectors.

Nonlinear core corrected pseudopotentials

USPs with nonlinear core correction (NLCC) are provided 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). On the other hand, 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 dropdown 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 the More... button to open the CASTEP Electronic Options dialog.
  4. Select the Potentials tab.
  5. Set the Pseudopotential for each element from the dropdown list which appears in the appropriate grid cell when it is edited (for example, for Mg the following pseudopotentials would be available Mg_00.usp, Mg_00PBE.usp, Mg_00PW91.usp, Mg_00.recpot).
  6. Click the View button to display the selected pseudopotential scheme.

Pseudopotential database

Pseudopotentials are stored as ASCII text files in the folder share\Resources\Quantum\CASTEP\Potentials. The headers of these files contain useful information about:

This information might be useful when deciding which potential to use, or when writing up the results of a calculation. In addition, core radii can be used 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

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