For the CASTEP Community Documentation and tutorial site see https://castep-docs.github.io/castep-docs/
CASTEP has a built-in keyword help system. To get more information on CASTEP use:
(where "castep" should be replaced with "castep.serial" for the serial version of CASTEP, or "castep,mpi" for the parallel version.)
Alternatively you can search for help on a particular topic
castep -help search <keyword>
where <keyword> is any text you'd like to search for in CASTEP's internal help system.
To list all of CASTEP's input parameters (in both the param and cell input files), use:
castep -help all