Starting from the square configuration of C 9 with C3(4i) at x,x,½ transformed to C1(4h) at x,x,0 (brown spheres), the structure was completed with C2(8i) (white spheres) and removal of the corner atoms, then subjected to full geometry optimization. The crystal data are given in Table . The structure of C 12 shown in Fig. now consists only of edge-sharing C4 tetrahedra.