For 1-Eu, a first calculation considering the ground spin octet and all state-averaged spin sextets, reveals four Kramers doublets (KDs) well-isolated from the rest of the spin-orbit coupled states (Table ), indicating that the magnetic response of the compound is dominated by these low-lying states. Inspection of these KDs wavefunctions reveals that they are made up of linear combinations of the ground 8 S7/2 free ion term and the states coming from the first excited 6 P term only, which allows us to simplify the calculation and consider one S=7/2 and three S=5/2 states. This in turn permits including dynamic electronic correlation effects by means of the extended multi-state (XMS) CASPT2 formalism (see Table for details on the different approaches). Thus, the interaction between the ground