This keyword controls the number of extra bands at each k-point in addition to the number of occupied bands, when performing a band structure calculation.
It is not possible to have both the BS_NBANDS keyword and either the BS_NEXTRA_BANDS or BS_PERC_EXTRA_BANDS keywords present in the same input file.
The default value corresponds to a calculation of the band structure for the valence band only. To calculate band structure for the conduction band use positive values of BS_NEXTRA_BANDS.
BS_NEXTRA_BANDS : 12
CASTEP keyword glossary
CASTEP parameters keywords