Solid ${\mathrm{C}}_{36}:$ Crystal structures, formation, and effects of doping

We employ plane-wave pseudopotential density functional calculations to show that the lowest energy $D_{6h}\hspace{0.5em}{\mathrm{C}}_{36}$ crystal is a highly bonded network of hexagonal planes of ${\mathrm{C}}_{36}$ units with $\mathrm{AB}$ stacking. This crystal is significantly more dense and lower in energy than previously proposed structures. We demonstrate that Na is the largest alkali atom that can be intercalated into the crystal structure without causing severe structural distortion. Further, we predict the reaction pathway to form a neutral ${\mathrm{C}}_{36}$ dimer to be barrierless, while negatively charged ${\mathrm{C}}_{36}$ molecules are less likely to bond due to a substantial barrier of formation.