Structural metastability of endohedral silicon fullerenes

Endohedrally doped Si-20 fullerenes appear as appealing building blocks for nanoscale materials. We investigate their structural stability with an unbiased and systematic global geometry optimization method within density-functional theory. For a wide range of metal-doping atoms, it was sufficient to explore the Born-Oppenheimer surface for only a moderate number of local minima to find structures that clearly differ from the initial endohedral cages but are considerably more favorable in terms of energy. Previously proposed structures are thus all metastable.