Cubic SiCN structure templates: The structure of cubic SiCN (space group F-43m, N° 216) is shown in Fig. resembling fluorite-type is built of face centered Si substructure with C and N occupying cube corners . Replacing N by C to model cubic SiC 2 for the sake of comparison in present work leads to higher symmetry fluorite-type (space group Fm-3m, N° 225) shown in Fig. . Both cubic SiCN and SiC 2 were submitted to unconstrained geometry optimization calculations of the energy. The numerical results of the cohesive energies obtained by subtracting the atomic contributions from the total electronic energy are shown in the first lines of Table .