Octacarbon C 8 with cfc topology structure shown in Fig. belongs to hexagonal space group P6 3 /mmc (No. 194) with two different 4-fold Wyckoff positions for carbon. Table shows that the presently calculated structure parameters of C 8 are in good agreement with the literature data given in brackets. Note that lonsdaleite C 4 (lon topology) crystallizing in the same space group has a single 4-fold carbon position, (4f) 2/3, 1/3, 0.06250, with lattice parameters a = 2.52 Å and c = 4.12 Å . The calculated atom-averaged cohesive energy of C 8 is the same as that of diamond and lonsdaleite, i.e. E coh /atom = -2.49 eV. In both space groups the interatomic distance d(C-C) is the same, 1.54 Å.