Significant changes in the binding energy are predicted also upon a spin transition involving a change in the occupation ). The Bader charge analysis shows a charge transfer of around 0.25 electrons from the metal to the ligands for Fe(II) and Fe(III) in absence of adsorbed molecules. We illustrate this change in Fig. where the difference in total electronic density is computed between S=2 and S=1 of the Fe(II)(CNH)4 complex, bound to CO. Because the electronic configuration changes from (dyz) 2 (d x 2 -y 2 ) 0 for S=1 to (dyz) 1 (d x 2 -y 2 ) 1 for S=2, a fraction of electronic charge density migrates from the iron to the ligands consistent with a d x 2 -y 2 orbital exhibiting a molecular character with some localization on the ligand (see Fig. ). This charge redistribution yields an increase in the positive charge of the metal which enhances the strength of the electrostatic component of the interaction with the guest molecule. Consistently with this analysis, the MOF-gas binding energy increases and the bond distances decrease (see Table ). Overall, the computed ∆E bind,H-L are smaller compared to the case where the spin state transition involves a change in the occupation of the d z 2 as shown in Fig. (upper panel). The whole mechanism is illustrated using a molecular orbital picture in Fig. for CO2 bound on Fe(II)(CNH)4. For S=0, the d z 2 is empty and the binding energy is the largest with E bind =0.960 eV and the metal-CO2 bond length of 2.06 Å. At intermediate spin, S=1, the d z 2 is singly occupied and the binding energy drops to 0.487 eV and the bond lengths lengthen to 2.42 Å. At S=2, the d z 2 is singly occupied and the d x 2 -y 2 changes from empty to singly occupied. Due to the enhancement of the electrostatic contribution to the total binding mechanism, the binding energy increases with respect to S=1 and becomes E bind =0.698 eV with a shortening of the metal-gas bond length to 2.24 Å. This analysis agrees with previous studies reporting a change in binding energy upon change in the occupation of d x 2 -y 2 achieved by changing the metal ion in high spin configuration. Specifically, an increase in binding energy was predicted from Cr to Mn and from Cu to Zn, as expected from an increase in the electrostatic charge of the ion. Table . Comparison between the binding energies computed using the PBE+D3+U and PBE+D3[U]. The stronger the binding in LS, the larger the occupation of the d z 2 and consequently the bias in the binding energy introduced by the Hubbard term.