On the multi-reference nature of plutonium oxides: PuO 2 2+ , PuO 2 , PuO 3 and PuO 2 (OH) 2

Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. Conventional multi-reference methods can treat active spaces that are often at the upper limit of what is required for a proper treatment of species with complex electronic structures, leaving no room for verifying their …