The fourth single component system, TTBI (Fig. ), was chosen as a more extreme test for the location of important higher energy crystal structures. TTBI forms four microporous polymorphs, named α, β, γ and δ, ranging from 46.4 to 92.1 kJ mol -1 (according to the FIT + multipoles force field) above the densely-packed global lattice energy minimum structure. These structures lie well outside the usual energetic range of polymorphism. They are accessed experimentally because they crystallize with solvent filling their pores and are stable to solvent removal, presumably as deep local energy minima.