This keyword controls the percentage of extra bands at each k-point in addition to the number of occupied bands, when performing an optical spectrum calculation.
It is not possible to have both the OPTICS_NBANDS keyword and either the OPTICS_NEXTRA_BANDS or OPTICS_PERC_EXTRA_BANDS keywords present in the same input file.
The default value corresponds to a calculation of the optical spectrum for the valence band only. To calculate optical spectra for the conduction band use positive values of OPTICS_PERC_EXTRA_BANDS.
OPTICS_PERC_EXTRA_BANDS : 60.0
CASTEP keyword glossary
CASTEP parameters keywords