Another interesting remark is about the potential coalescence of two ZnIm 4 clusters (the most stable Zn1 form) to yield Zn 2 Im 7 (the most stable Zn2 form). The calculated free energy of the reaction ZnIm 4 +ZnIm 4 → Zn 2 Im 7 +Im is actually positive (+0.17 eV), so there is no thermodynamic driving force for this reaction to occur. A plausible route would be the reaction ZnIm 4 + ZnIm 3 → Zn 2 Im 7 , with a favorable free-energy balance (-0.28 eV). This suggests an active role of the [ZnIm 3 ] -cluster in the pool of aggregation precursors of ZIFs, which would be in consonance with postulations from previous studies for analogous Co 2+ -imidazolate ZIF frameworks, and from recent studies, which found that tri-coordinated species are still present over long simulation times. Another possible reaction would be ZnIm 4 +ZnIm 2 → Zn 2 Im 6 , followed by the exchange of coordinating ethanol or nitrate molecules by an imidazolate molecule to form Zn 2 Im 7 (free energy balance of -0.95 eV). This reaction has a higher free energy, probably due to the lower stability of the ZnIm 2 cluster with respect to the ZnIm 3 cluster. Zn3 clusters: While Zn3 ring structures are the most stable in the pool of precursor Zn 3 Im n X m clusters (n = 3-8, m = 0-3), the full four-fold coordination of three Zn 2+ cations with imidazolate anions is marginally more favorable in a cyclic [Zn 3 Im 9 ] -3 cluster (-4.74 eV) over the linear [Zn 3 Im 10 ] -4 cluster (-4.71 eV) ones. This again points to the effect of deformation of the local structure, since the stress induced by the constraint of the formation of cyclic structures reduces the stabilization gained by the higher coordination achieved when cycles are formed. The opening of the ring could be induced in the last stage upon the incorporation of an additional imidazolate anion to the Zn 3 Im 9 cluster, to form the linear Zn 3 Im 10 cluster, but this does not seem to be the case (∆g = + 0.08 eV). Rather, this latter cluster may be mainly produced through the aggregation reaction [Zn 2 From the structures reviewed so far, it can be inferred that, despite the tendency of Zn cations to eventually bind to four imidazolate molecules forming a tetrahedron, in a similar fashion as in ZIFs crystal structures, most of the reactions that drive cluster growth involve the presence of three-fold coordinated moieties. For instance, the low free energies of the [ZnIm 3 ] -and [Zn 2 Im 5 ] -clusters (-4.41 eV and -4.32 eV respectively) suggest that a large number of these threecoordinated, planar species are present in solution during the initial stages of aggregation.