In the present article, we proffer an explanation for the origin of single molecule magnetism in [Cu^IILTb^III(hfac)₂]₂, a member of the novel class of mixed transition−lanthanide metal clusters. The theoretical model takes into account the crystal field acting on the Tb^III ion as well as the exchange interaction between the Tb^III and Cu^II ions and provides a good account of the low-lying energy levels, rigorously measured by inelastic neutron scattering and magnetic data. We demonstate that the single molecule magnet (SMM) behavior is not a sole consequence of the low-lying levels of the lanthanide metal ion but a property of the tetranuclear cluster itself. The energy levels are shown to increase with the decrease of the mean value of the Z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is wholly consistent with the observed SMM behavior. On the basis of this study, recommendations are formulated on how the SMM properties for nd−4f clusters may be further improved.