This keyword determines the number of geometry optimization steps for which the total spin is fixed. If GEOM_SPIN_FIX is less than 0, the spin will be fixed to the value found at the end of the SCF calculation for the initial structure for the duration of the calculation.
This parameter is only used if FIX_OCCUPANCY = FALSE. So for insulators the spin is fixed regardless of the value of GEOM_SPIN_FIX.
The default value for this parameter is 0, so spin is allowed to vary.
GEOM_SPIN_FIX : 5
CASTEP keyword glossary
CASTEP parameters keywords