BCC is not used by MOPAC at all, but instead is used to inform programs MAKPOL and BZ about the nature of the solid being studied.
BCC should be specified if all odd unit cells are omitted, e.g. unit cells (0,0,1), (1,2,4), and (3,2,2) are excluded, but (0,0,0), (1,3,2), and (2,2,2) are included. An example is provided by diamond where the unit cell consists of two atoms. BCC is used during the generation of Large Unit Cells using program MAKPOL and also used in k-space or Brillouin zone work using program BZ. This applies only to layer and solid systems.