The electronic band structures are shown in Fig. . Along the x-axis, the bands develop along the main lines of the tetragonal BZ. Along the y-axis, the zero energy is with respect to the top of the valence band (VB), i.e. E V . Like C 4 , both C 9 and C 12 allotropes are insulating with large band gaps, close to 5 eV. This behavior is an indication of the closeness of these two allotropes to diamond.