This deviation can likely be attributed to higher 1p1h excitations, which include dynamic charge and spin polarization effects that are implicitly captured in the NEVPT2 calculations Table : Ferromagnetic and antiferromagnetic contributions (J AF,σ/F ) for the equilibrium geometry of the copper complex, calculated with the Cu -O -Cu angle scan approach using CASSCF and NEVPT2 calculations with (2,2) and (8,5) active spaces.