As a first assessment, the trend of cohesive energy per atom obtained after full geometry relaxation onto ground state is a decrease from E total/atom (tet-C 4 ) = -2.4 eV to E total/atom (tet-C 6 ) = -1.7 eV. Such trend is explained by the loss of the homogeneous 3D corner sharing C4 tetrahedra observed in diamond-like tet-C 4 in contrast to tet-C 6 . Nevertheless, tet-C 6 remains cohesive, and as shown below, it is stable both mechanically (elastic properties) and dynamically (phonons).