The Pu 4+ 5f 4 ion in a cubic ligand field is expected to have a non-magnetic nondegenerate electronic ground state corresponding to the ligand field and spin-orbit split 5f component Γ 1 , based on the spin-orbit splitting of lowest t 1u orbital level. Magnetic measurements for PuO 2 have confirmed this. Likewise, O NMR spectroscopy and a rare Pu NMR measurement also point to a non-magnetic ground state, consistent with the single-ion description of the system's magnetism.