SIESTA uses pseudopotentials to represent the electron-ion interaction (as do most plane-wave codes and in contrast to so-called "all-electron" programs). In particular, the pseudopotentials are of the "norm-conserving" kind.
The released versions of SIESTA use pseudopotential files in the 'Froyen' (psf) format. These pseudopotentials can be generated and tested by a number of programs:
For non-academic users, or for those looking for more pseudopotential options, we recommend a visit to the Virtual Vault for Pseudopotentials. Use one of the pseudopotential generators listed as providing psf output compatible with Siesta (such as APE) and follow the proper procedures for testing.
Remember that all pseudopotentials should be thoroughly tested before using them. (See the standard literature on pseudopotentials and the user manuals for ATOM and other generation codes for more information on testing).
It follows that databases of pseudopotentials should be used with care. Unless an explicit description of the generation conditions and testing procedures is given, a pseudopotential found in a database should be treated as just a starting point for further refinement. An example of this kind of database is the one generated by just translating into the .psf format a set of pseudopotentials generated with the fhi98pp program for the Abinit project. It can be found here. We stress that these pseudopotentials should not be used blindly.
In contrast, curated pseudopotential databases offer a higher level of testing and quality assurance. For example, the PseudoDojo is an open source project for developing and systematically testing pseudopotentials. These are generated with the multi-projector norm-conserving generator ONCVPSP, tested with seven different batteries of tests with ABINIT, and made available in a web-based database in several formats.
Users of SIESTA willing to try these pseudopotentials can use an experimental but soon to be merged version of SIESTA with support for the new PSML format.