Freedom and Constraints of a Metal Atom Encapsulated in Fullerene Cages

Ab initio molecular dynamics applied to fullerenes with La and Y inside reveals an unforeseen picture of these fascinating host-guest complexes and answers long-standing basic questions on the metal-cage interaction. Adsorption sites of the metal atom, nature, and time scale of its motion are identified. They depend on the cage structure in a simple way, applicable to any fullerenelike host with electron donors inside. Results for (La, Y)@${\mathrm{C}}_{82}$ are consistent with experiment and suggest its potential as electronic molecular switch. Light is shed onto the strikingly poorer stability of (La, Y)@${\mathrm{C}}_{80}$.