CASSCF method was applied for further exploring the nature of the metal-metal bonding in the [Re2-Model] 1-molecule. A CAS(9,8) active space was adopted, including the eight metal-based orbitals and nine electrons distributed among them using the DFT-OLYP geometry (Re-Re bond distance of 2.22 Å). The active space natural orbitals with occupations of the [Re2-Model] 1-are appeared in Fig. . The