This keyword determines the number of electronic iterations for which the total spin is fixed. If SPIN_FIX is less than 0, the spin will be fixed for the duration of the calculation.
This keyword only effects the behavior of the electronic minimizer in the initial SCF calculation. There is a separate keyword, GEOM_SPIN_FIX, which should be used to fix the spin of the system during geometry optimization.
This parameter is only used if FIX_OCCUPANCY : FALSE. So, for insulators the spin is fixed regardless of the value of SPIN_FIX.
SPIN_FIX : 5
CASTEP keyword glossary
CASTEP parameters keywords