The present MC-WFT calculations produce the same electronic states as obtained previously, with multiplicities of 1 (A or Γ 1 ), 3 (T 1 or Γ 4 ), 3 (T 2 or Γ 5 ) and 2 (E or Γ 3 ) in increasing order of energy, collectively representing the 9 components of the 5 I 4 ground level of the ion split and perturbed by the ligand field. The energy gap ΔE between the A ground state and the T 1 first excited state is of crucial importance for the observed susceptibility and has been measured at 0.123 eV (123 meV). Table shows that the present CAS(4,7) calculations including the SO interaction reproduce the experimental ΔE nearly quantitatively with the embedded cluster model. The dynamic correlation effects on the state energies at the NEVPT2 level lead to a slight destabilization of the low-energy electronic excited states as compared to the CASSCF-SO calculations. The NEVPT2 results are in very close agreement with previously (12,16) restricted active-space self-consistent field calculations with SO coupling. d Within 10 meV, the energies were the same for the (4,7) and (4,12) active spaces.