“…Actinides are present in nuclear and radioactive processes; therefore, understanding their electronic structure is very important. , Being able to accurately describe the behavior of f-electrons in computations, including strong correlation effects, spin–orbit coupling, and multiplet complexity, is a challenge. This is especially true because actinides possess a strong crystal field and spin–orbit coupling (SOC). − A multiconfiguration treatment, including SOC, is indispensable for calculations on actinides. Active-space self-consistent field calculations account for static correlation (near degeneracy effect) and are usually followed by a perturbative (second-order) treatment that recovers dynamic correlation, which is very important for actinide molecules to correctly describe metal–ligand covalency , because the 5f valence orbitals in actinides are diffused and hence exert a strong ligand field. − However, multireference second-order perturbation theory (MRPT2) methods are very expensive when the active space is large (e.g., beyond the 5f shell) and also when many electronic states are computed, using state average procedures. ,, …”