Covalency in the actinide dioxides: Systematic study of the electronic properties using screened hybrid density functional theory

We present the first systematic study of the electronic properties of the AnO(2) series, An=Th-Es, using screened hybrid density functional theory. In contrast to local or semilocal functionals, this approach has been demonstrated to capture the strong electron correlation and localized Mott-insulating behavior observed in early dioxides such as UO2. One might expect later members of the series to show even more localized character as the f orbital radial extent decreases with increasing Z. However, we find two interesting features in the calculations: a 5f-O 2p orbital energy degeneracy, which leads to significant orbital mixing and covalency in the intermediate region (PuO2-CmO2), and a strong Hund's rule exchange opposing spin-orbit coupling, which yields an unexpected ground state in CmO2.