To rationalize differences in needed U parameters, the sensitivity of the adsorption energy to the U parameter can be estimated by the difference in the PBE fractionality of the bare SBU and SBU/gas-molecule system, ΔTr[𝒏(𝟏 -𝒏)], which is directly proportional to the Hubbard energy correction term. Here, 𝒏 is the occupation matrix of the orbitals where the U parameter is applied, i.e., the metal 3d orbitals. For each orbital, the term 𝑛(1 -𝑛) takes on a value of 0 when the orbital is either completely filled or empty. Its value increases to a maximum of 0.25 when the orbital is half-filled, the regime in which the system is maximally impacted by corrections from the U parameter. The PBE fractionality analysis shows that, in general, the adsorption energy of CO2 exhibits the least sensitivity, whereas the adsorption energy of O2 is the most sensitive (Figure ).