The results of a computational study with multiconfigurational quantum chemical methods on actinide monoxides (AnO) and dioxides (AnO₂) for An = Th, Pa, U, Np, Pu, Am, and Cm, are presented. First and second ionization energies were determined and compared with experimental values, when available. The trend along the series is analyzed in terms of the electronic configurations of the various species. The agreement with experiment is excellent in most cases. Of particular interest is the first ionization of PuO₂. We applied cutting-edge theoretical methods to refine the ionization energy, but our computed data fall in the range of ~6 eV and not in the ~7 eV region as the experiment dictates. Such a system requires further computational and experimental attention.

The ground and excited states of neutral and cationic PuO and PuO₂ have been studied with multiconfigurational quantum chemical methods followed by second order perturbation theory, the CASSCF/CASPT2 method. Scalar relativistic effects and spin–orbit coupling have been included in the treatment. As literature values for the ionization energy of PuO₂ are in the wide range of ~6.6 eV to ~10.1 eV, a central goal of the computations was to resolve these discrepancies; the theoretical results indicate that the ionization energy is near the lower end of this range. The calculated ionization energies for PuO, PuO⁺ and PuO₂⁺ are in good agreement with the experimental values.