The photoluminescence properties and field-induced single-molecule-magnetic behavior of a novel erbium(III) β-diketonate homodinuclear complex, [Er₂(nd)₆(μ-bpm)] (nd = 2,4-nonanedione and bpm = 2,2′-bipyrimidine), are presented. On the basis of structural data, ab initio calculations using the SO-CASSCF method have been performed so as to gain insight into the spin-relaxation mechanism, giving evidence of the nature of the m_J ground state and the orientation of the related magnetic anisotropy axis. The overlap integrals between the natural spin orbitals (NSOs) of the mononuclear fragments, related to the magnitude of the antiferromagnetic exchange, show that the two fragments have no magnetic interaction.