We gratefully acknowledge financial support from the France-Canada Research Fund, Natural Sciences and Engineering Research Council of Canada (NSERC). This research was supported by the NEEDS environment project (TEMPO, "ThErmodynamique des Molécules de PlutOnium") funded in 2017. We acknowledge support by the French government through the Program "Investissement d'avenir" (LABEX CaPPA / ANR-11-LABX-0005-01 and I-SITE ULNE / ANR-16-IDEX-0004 ULNE), as well as by the Ministry of Higher Education and Research, Hauts de France council and European Regional Development Fund (ERDF) through the Contrat de Projets Etat-Region (CPER CLIMIBIO). Furthermore, this work was granted access to the HPC resources of [CINES/IDRIS/TGCC] under the allocation 2016-2019 [x2016081859 and A0010801859, A0030801859, A0050800244] made by GENCI. We also acknowledge Małgorzata Olejniczak for her exploratory work on the benchmarking of relativistic DFT and MS-CASPT2 calculations. We further thank Sidi Souvi and Marc Barrachin for the ver...