Pure plutonium is an exotic metal that is poorly understood. The α-Pu phase (zero-temperature ground state) is believed to have mainly itinerant 5f electrons. However, δ phase (fcc) is believed to be in neither limit (neither completely itinerant nor localized), that we may describe as partially localized. How to understand this phase transformation, with its associated large volume per atom expansion, has been an exciting topic of discussion within band-structure theory and related correlated-electron extensions to it for several decades now. A correct theory should get both limits right, and is a strong constraint on the theoretical approaches that are used. The problem with including the α phase is its complicated crystal structure, which is monoclinic and involves 16 atoms per unit cell.
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