The mean µ, standard deviation σ , and MAE for the %errors affecting the energy gaps within the lowest energy spin-orbit multiplet for Ln-acac 3 -(H 2 O) 2 complexes, estimated using different approximations to the Breit-Pauli Hamiltonian, are represented in Figure (see also Table in the Supporting Information (SI)). In the following, MAE = |µ| if not explicitly reported.