U@C28: the electronic structure induced by the 32-electron principle.

First principles calculations show that the neutral U@C28 has a (cage)(2) ground state with Td symmetry instead of the long believed (5f)(1)(cage)(1) ground state with D2 symmetry. Its 34 valence electrons preferentially obey the 32-electron principle which fills all the s-, p-, d-, and f-type valence shells of the uranium atom. The remaining two valence electrons cannot break the electronic configuration and thus are located on the cage.