The castep_relax script

This is script supplied by the airss package. The main idea is to restart the DFT calculations every certain number of geometry step, in order to update the basis set when running in the fixed basis set mode with variable unit cell. By default, the script does several restarts with very small number of ionic steps, as it is expected that the volume could change significantly in the very begining. Afterwards, the geom_max_iter setting in the param is respected. The optimisation is terminated if CASTEP reports that the optimization is successful in two consecutive restats, of if the maximum number of steps has been reached.

The input syntex is:

castep_relax <maxit> <exe> <sim> <symm> <seed>

These arguments are explained as below:

• <maxit> is the maximum total number of geometry optimisation steps among all restarts. The optimisation is assumed to be finished if this number of steps has been reached.
• <exe> is the launch command for running CASTEP, including the MPI launch command and its arguments.
• <sim> is a switch to enable check if the same structure has been found before. It should be disabled by setting it to 0 in DISP, as each calculation is run under a different directory.
• If the <symm> switch is set to 1 the structure will be symmetrised on-the-fly during the restarts. It is typically turned off.
• <seed> is the name of the seed for CASTEP.

When to use it

DISP invoke this script during the search and relaxation tasks to perform the geomettry optimisation. Typically, the user does not need to use this script manually, although it can useful when testing the parameters or for performing manual relaxations of reference structures.

In the standard AIRSS search, this script is invoked by airss.pl internally for CASTEP relaxation.