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
.uspcc extension, norm-conserving files have a
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
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.
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
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.
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.
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
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
To change pseudopotential selection for a given element
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.
Setting electronic options
Electronic tab - CASTEP Calculation dialog
Potentials tab - CASTEP Electronic Options dialog