The bands (blue lines) develop along the main directions of the tetragonal Brillouin zone. The zero energy along the vertical axis is considered with respect to the Fermi level E F , since all three allotropes C 5 , C 6 and C 7 are metallic-like with no energy gap separating the valence band VB from the conduction band CB. Such behavior can be explained by the delocalized nature of the π electrons resulting from the presence of C(sp 2 ) atoms of the C=C and C=C=C subunits.