The addition of a central carbon atom to tet-C 6 results in C 7 stoichiometry. The fully geometryoptimized lattice parameters are given in Table . There are now 4 different Wyckoff positions in the primitive tetragonal space group P-4m2, No. 115. The topological analysis led to the assignment of tfa topology as in the case of tet-C 5 . The structure of tet-C 7 is shown in Fig. , with the aligned C=C=C connecting the tetrahedra within the cell. As in C 5 and C 6 , two adjacent cells are connected via C2-C2. However, tet-C 7 has the exception of having both C=C ene and C=C=C propadiene subunits. The cohesive energy E coh /atom = -1.69 eV is lower than that of tet-C 6 .