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.
Versions of SIESTA before 5.0 use pseudopotential files in the 'Froyen' (psf) format. These pseudopotentials can be generated and tested by a number of programs, which include:
The Atomic Pseudopotential Engine (APE) is a computer package designed to generate and test norm-conserving pseudopotentials within Density Functional Theory. The generated pseudopotentials can be either non-relativistic, scalar relativistic or fully relativistic and can explicitly include semi-core states. A wide range of exchange-correlation functionals is included through Libxc.
We used to provide a database of pseudopotentials in psf format, but we no longer do, as their quality was uneven. A database of pseudopotentials is still available at the Virtual Vault for Pseudopotentials site, but please remember that all pseudopotentials should be thoroughly tested before use. (See the standard literature on pseudopotentials and the user manuals for ATOM and other generation codes for more information on testing).
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.