We discuss first the M=Fe case. The ground state of the bare MOF is S=1. Upon coordination of CO2, the ∆EI-L (S=1 and S=0) increases as a result of the destabilization of the d z 2 by the interaction with the molecule but it does not yield a change of sign owing to the weak field ligand of CO2. For the other molecules HS also correspond to S=1 and the strong interaction with the Fe center computed in S=0, i.e. 1.007 eV (H2), 1.223 eV (N2), and 2.344 eV (CO), allows to stabilize LS yielding positive values of ∆EH-L that correlate well with the strength of the binding. For CO, the interaction is so strong that we could only converge S=2 (and not S=1) preventing us from computing the values of adiabatic energy difference and the binding energy in the IS case.