In Figure we show the geometries of a selection of the largest clusters included in this study, with a number of cations ranging from 5 to 144. The energies of all the clusters are reported in Table . We find that fully coordinated Zn clusters have energies ranging between -4.00 and -5.54 eV (per Zn cation) while clusters with undercoordinated Zn atoms are less stable. There is an unexpected high energy for the cluster sod-Zn 24 (with all 4-fold Zn cations and forming 6MR), which can be ascribed to the large cavity inside the cluster, making it less stable, as a large number of solvent molecules must be excluded to make space for it, so that this high value might be an artifact of the method employed to calculate the energy. The formation of bulk-like clusters can occur after the clusters have achieved a critical size, while clusters smaller than that size feature non bulk-like geometries. The transition from small clusters to bulk-like clusters via intermediate polymorphs (structures which in bulk phase are metastable) is commonly observed in Zn-based materials, such as ZnS and ZnO, and other inorganic materials (TiO 2 , etc.), 67-69 and it is an example of Ostwald's Rule, which states that the first crystal structure formed would be the least stable polymorph, and it also applies to MOFs. It is only after a critical nanoparticle size is achieved that the stable crystalline phase appears. Indeed, for the larger cluster sod-Zn 144 , the formation energy is much lower (-5.49 eV) than that of the sod-Zn 2 4 cluster (-3.83 eV), and that of the periodic crystal structure SALEM-2 (sodalite) is -5.86 eV, while the most stable crystal structure is zni (-6.18 eV, see below for a discussion). It is interesting that the relatively stable large Zn 144 cluster (with both 4 and 6MRs) contains six relatively unstable Zn 24 clusters. But the assembly of the Zn 24 clusters results in further 4MRs being generated, which appears to stabilize the larger cluster. The role of the assembly of small units in forming large pore zeolites was postulated three decades ago, but not seen before in MOFs. Another salient point from Figure is the large stability of the (41, 88) cluster (formation energy -5.54 eV). This structure was generated by MD simulations at high temperature; the resulting structure is a continuous random network containing most tetracoordinated and some undercoordinated Zn atoms. The core of this cluster is dense, having no internal porosity, which greatly increases stability, overcoming the penalty associated with the (small) number of undercoordinated Zn cations present in the structure. This is the first cluster we find to be relatively more stable (per Zn cation) than ZnIm 4 . It is important to note that this cluster contains a z = 41 value considerably lower than the others with similar energies (60 and 144). This is in agreement with the experimental observation that glassy clusters form before crystalline clusters become more stable. Likewise, it also explains the formation of glassy ZIFs under specific reaction conditions, 71 as was found by Balestra and Semino 25 by computer simulations.