Encapsulation of an f-block metal atom/ion to enhance the stability of C-20 with the I-h symmetry

Based on the density functional theory, the geometric and electronic structures, chemical stability, and bonding properties of the endohedral metallofullerenes, M@C-20 (M = Eu3-, Am3-, Gd2-, Cm2-, Tb-, Bk-, Dy, Cf, Ho+, Es+, Er2+, Fm2+, Tm3+, Md3+, Yb4+, No4+, Lu5+, and Lr5+), were investigated. Through encapsulation of an f-block metal atom/ion with 12 valence electrons, the bare C-20 cage with the D-2h point group could be stabilized to a highly symmetrical I-h structure. The calculated values of HOMO-LUMO energy gaps using the B3lYP and BHHLYP functionals ranged from 2.22 to 5.39 eV and from 3.89 to 7.95 eV, respectively. The stability of these metal-encapsulated clusters can be attributed to the 32-electron rule, where the central metal atom\u27s orbitals strongly participated in the t(2u), g(u), t(1u), h(g), and a(g) valence molecular orbitals