After full geometry relaxation, the resulting structure is shown in Fig. with colored spheres corresponding to the three carbon sites given with the coordinates in Table , i.e. with well determined z = 0.854 and zʹ = 0.314. The topology of tet-C 5 is found to be 3,4^2T1-CA, which is not documented in the SACADA database, while the template tet-C 4 has a dia topology. Now the cohesive energy per atom: E coh /atom = -2.06 eV is lower than that of tet-C 4 , which is characterized by a large cohesive energy E coh /atom = -2.49 eV (the same as in diamond).