Ab initio calculations of heavy-actinide hexahalide compounds: do these heavy actinides behave like their isoelectronic lanthanide analogues?

Research on heavy actinides has experienced an increased interest in the last few years due to new synthetic techniques and recent technological advances that have allowed for obtaining important information even from very small samples. This area presents challenges not only from the experimental point of view but also from the theoretical perspective. This work deals with a multiconfigurational CASSCF and NEVPT2 benchmark study based on a two-step methodology that considers first correlation effects and then the spin-orbit coupling applied to berkelium (Bk), californium (Cf), einsteinium (Es) and fermium (Fm) hexahalides. Optical properties, such as f → d transitions and crystal-field parameters, have been calculated and rationalized. The results for these trivalent actinides indicate that the electronic structure of the low-lying states is reproduced accurately with small basis sets. The ground-state multiplets are isolated, in the same manner as their isoelectronic lanthanide counterparts. In the case of tetravalent berkelium, the picture is different regarding the electronic structure where crystal-field theory fails due to considerable ligand-to-metal charge transfer contributions to the ground state.