Total energy calculations of the C70 fullerene nano-cage doped with transition metals, (TM¼Fe and Co atoms), endohedrally, exohedrally, and substitutionally were performed using density functional theory with the generalized gradient approximation along 18 different paths inside and outside of the fullerene. The most stable structures were determined with full geometry optimization near the minimum of the binding energy curves of all the examined paths. The results reveal that for all stable structures, the Co atom has a larger binding energy than the Fe atom. It is also found that for all complexes additional peaks contributed by TM-3d, 4s, and 4p states appear in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) gap of the host cluster. The mid-gap states are mainly due to the hybridization between TM-3d, 4s, and 4p orbitals and the cage p orbitals. The magnetic moment of endohedrally and exohedrally doped Fe and Co atoms in the C70 fullerene are preserved to some extent due to the interaction between the TM and C atoms of the cage, in contrast to the completely quenched magnetic moments of the Fe and Co atoms in the C69TM complex. Furthermore, Mulliken charge population analysis shows that overall charge transfer occurs from TM atom to the cage.

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