Theoretical calculations. Density functional theory calculations were performed to shed light into the coordination process between carvacrol and the Fe III oxo-trimeric core, and the formation of the mixedvalence Fe II /Fe III MIL-100(Fe) upon thermal activation. An oxo-centred tri-metallic cluster model with six benzoate ligands was extracted from the crystal structure of MIL-100(Fe), bearing two water molecules and a fluoride anion in the coordination environment of Fe III atoms for charge neutrality, and was optimized at the B3LYP/6-31G(d,p) level of theory in a high-spin configuration (Figure ). A preliminary evaluation of the thermodynamics of Fe III coordination bonds, indicates that insertion of carvacrol in MIL-100(Fe) is expected to occur by replacing a coordinative water molecule (Figure ), this process resulting energetically favoured (DE = -16.15 kcal/mol) (Figure ). The minimum-energy structure of the oxocentred trimetallic cluster indicates that the hydroxy group of carvacrol effectively coordinates with an Fe III atom, displaying a small interatomic distance of 2.29 Å (Figure ), similar to that calculated for a coordinating water molecule (2.25 Å). Non-covalent index (NCI) surfaces evidence a large number of weak but stabilizing dispersion interactions between the carvacrol moiety and the benzoate units of the cluster (green surfaces in Figure ), whereas the strong coordination bond between the hydroxy group of carvacrol and one Fe III is revealed as a localized bluish NCI surface (Figure inset and Figure ). A bigger cluster consisting of the pentagonal gate of MIL-100(Fe) was modelled to support the structural and stability properties predicted for the coordination of carvacrol. After geometry optimization, a carvacrol molecule was placed inside the pentagonal window to replace a coordinating water molecule (Figure ). Theoretical calculations at the B3LYP/6-31G(d,p) level indicate that the coordination of carvacrol is significantly more stable than with water (DE = -11.23 kcal/mol for the reaction of replacing a coordinating water by carvacrol), in good accord with that predicted for the smaller cluster model and in line with the experimental evidences. NCI surfaces confirm the presence of a large number of noncovalent interactions between the carvacrol molecule and the framework, along with a strong, localized interaction corresponding to the coordination bond with Fe III (see Figure ). Note that the pentagonal pore gate is predicted with a diameter of 5.5 Å, which nicely fits the carvacrol size of 5.0 Å calculated without considering the hydroxy group (see light yellow region in Figure ). The hexagonal pore gate, which is of about 8.6 Å, is therefore expected to bear and diffuse the carvacrol along the framework easily.