We report the computed equilibrium geometry of percubylcubane which serves as a model for supercubane. Supercubane is a very dense form of carbon thought to be composed of cubane subunits. Results for cubane, cubylcubane and percubylcubane are presented. Calculations were carried out for cubane and cubylcubane with Hartree-Fock and MP2 wavefunctions using the STO-3G and 6-31G* basis sets, respectively. A calculation was carried out for percubylcubane with a Hartree-Fock wavefunction and the STO-3G basis set. A comparison of the calculated geometries of cubane and cubylcubane with the corresponding experimental data yields correction factors for the equilibrium geometry computed for percubylcubane. Percubylcubane was determined to have a structure with 0 symmetry. The dihedral angles for the intercube bonds are rotated approximately 15 degrees from the previously postulated Burdett and Lee structure. The 15 degrees rotation occurs primarily for steric reasons. This rotation may also explain why supercubane is more dense than would be expected from the Burdett and Lee structure. Using our structure for percubylcubane as a non-primitive unit cell, we propose an alternative structure for supercubane. The primitive unit cell of our proposed structure is of Sg symmetry and the crystal group is <R(3)over bar> (C-3i2, No. 148).