The final molecule investigated is TTBI (Fig. ), which has four known polymorphs that occupy high energy regions of the crystal structure landscape. The positions of the observed structures are indicated on the crystal energy landscape (Fig. ), showing that the observed structures fall outside the usual energetic range of polymorphism. The proposed explanation for the observation that these very high energy structures can be isolated as stable materials is that they occupy deep, isolated regions of the energy landscape, which is hinted at by the 'spikes' of structures Figure : Lattice energy vs density plot of the landscape of predicted crystal structures of TTBI from CSP (quasirandom sampling), with predictions performed in P 1 with four molecules in the unit cell. The experimentally observed structures are labelled α, β, γ and δ. The global energy minimum is labelled ε that fall below the bulk of structures on the energy-density landscape; α, β and γ are low energy structures within these spikes. We apply the threshold algorithm to add information to the CSP landscape and improve our understanding of the observed polymorphism of TTBI.