The polyhedral projection of the tet-C 5 structure (Fig. , center) shows four tetrahedra. More details are observed in the right panel of Fig. with a double cell along the c-direction, highlighting the C2-C2 bonds between the tetrahedra in these two cells. The C2-C2 segments form double bonds like C=C ene units, which will be further discussed in Section 4 relevant to the charge density projections.