Properties of small carbon clusters inside the ${\mathrm{C}}_{60}$ fullerene

We present the results of an atomic-scale simulation of the confinement of small carbon clusters inside icosahedral ${\mathrm{C}}_{60}$ fullerene. We carefully investigate the incorporation of various clusters into ${\mathrm{C}}_{60}$ including chains, rings, and double ring configurations, and have analyzed both the energetics and the resulting geometries. The calculations have been performed employing the density-functional-based tight-binding methodology within the self-consistent charge representation. We find that certain carbon cluster configurations that are unstable as free molecules become stabilized inside ${\mathrm{C}}_{60}.$ By adding single atoms into random positions inside the fullerene shell we establish an upper limit for the filling of ${\mathrm{C}}_{60}$ with carbon. When the number of atoms inside the fullerene exceeds ten we observe bonding to the surrounding shell and, hence, a gradual transition of the fullerene towards an ${\mathrm{sp}}^3$ rich but locally disordered carbon system.