  Endohedrally doped Si20 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.
