Subsequent geometry relaxation yielded a new 3D structure with C 5 stoichiometry (Fig. ). The crystal data with three different carbon positions and a symmetry reduction to P-6m2 (No. 187) are presented in Table . The topology is now lon, i.e. lonsdaleite-like. The bottom row shows the total energy after full geometry relaxation, which is found to be larger for tet-C 5 than for h-C 5 . The result can be translated into the average cohesive energy per atom, E coh /atom, which is obtained after subtracting the atomic energy of the carbon atom in a large box, i.e. -6.6 eV. Then the cohesive energy is -2.05 eV for tet-C 5 versus -1.90 eV for h-C 5 . Both values remain smaller than E coh /atom = -2.49 eV for diamond.