Using the indexes summarized in the tables shown in Figures 4, S6 and S7, we found the corresponding planes within the crystallographic structures. Interestingly, these planes do not follow the mathematical boundaries of the unit cell. In contrast, the concave facets correspond to planes defining the walls of the channels and can direct the crystal morphology. The crystals have a low, inhomogeneous distribution of molecular density in the unit cell, with voids comprising ~37% of the unit cell volume (calculated using the contact surface employing a spherical probe with a radius of 1.2 Å). As stated above, the 6-fold axis of the crystal runs through the center of a channel. Likewise, the larger 3-fold channels also center around special positions within the unit cell. Therefore, the low miller-index crystal planes connecting these lattice points necessarily cut through the channels relatively devoid of the molecular material (unlike close-packed molecular crystals). In the case of MOFs, the coordination chemistry driven attachments, combined with the inhomogeneous distribution of atoms in the unit cell around the channels, causes the growth fronts energetically and kinetically distinct for each of the different facets and facet portions: molecular attachment should be favored in the areas of the unit cell that are denser, causing an anisotropic evolution of the crystal facets, in our case the re-entrant crystal planes.