CASTEP has several internal libraries of pseudopotentials which can be selected using.

%block species_pot
%end_block species_pot

The default library is the latest ultrasoft set. The most common library you may wish to switch to is the latest set of Norm Conserving Pseudopotentials selected using the library NCP; some CASTEP functionality is only available for norm-conserving pseudopotentials.

In general, the pseudopotentials are designed to be accurate, even if it means they require a high plane-wave cut-off energy. For high throughput calculations and quick feasibility studies there is a set of ultrasoft pseudopotentials which is designed to be used at a plane-wave cut-off energy of 272 eV, and within this limitation has been made as accurate as possible across the periodic table (using a process known as qc-tuning). This library is selected by choosing QC5.