In this work we performed ab initio pseudopotential, density functional calculations of the structure and electronic properties of two hypothetical carbon structures with cubic symmetry: ${\mathrm{C}}_6$ bcc, body centered with 12 atoms per cubic unit cell, and ${\mathrm{C}}_{20}$ sc, simple cubic with 20 atoms per unit cell. The former is semiconducting with an indirect gap of $2.5\mathrm{eV}$—within the local density approximation to the exchange and correlation energy functional—while the latter is metallic. Both have similar zero-pressure densities of about one atom per $6.9{\mathrm{\AA }}^3$, which is intermediate between graphite and diamond, and similar bulk moduli of about $350\mathrm{GPa}$ at ambient conditions. Both are metastable with respect to graphite and diamond, and no phase transition is expected in the range of pressures studied.

• Received 18 April 2006

DOI:https://doi.org/10.1103/PhysRevB.74.172101