The basin containing the low density γ polymorph is separated by an even larger energy barrier from β, δ and ε. This energy barrier connecting the γ trajectory to the other Table : Lid energies at which transitions are located between γ, β, δ and ε in threshold sampling in space groups P 1 (upper right entries) and P 2 1 /c (lower left entries). Energies in italics along the diagonal are the calculated lattice energies of the four structures. All energies are in kJ mol -1 . three structures shows the largest difference between results in the two space groups. The lid energy at which γ connects to the others is -89.2 and -59.2 kJ mol -1 in P 1 and P 2 1 /c, respectively, representing a barrier of 80 or 110 kJ mol -1 for the transformation of γ into the global energy minimum ε or one of the other known polymorphs. Despite the large quantitative difference found when sampling with different symmetry constraints, the same qualitative picture emerges: globally, γ is separated from the main pocket formed by ε, β and δ, occupying its own funnel on the connectivity graph. This finding agrees with the surprisingly good reported thermal stability of the γ polymorph, despite its exceptionally low density. The α polymorph of TTBI crystallizes in space group P 4 2 /m with four molecules in the unit cell and cannot be represented in the unit cells included in the P 1 and P 2 1 /c simulations. To include α, we performed a further set of simulations in a P 1 unit cell containing four symmetryindependent molecules, starting trajectories from all five structures (α, β, γ, δ, ε) and attempting 5,000 steps per lid energy. Due to the wide range of initial energies, the number of lids varied between trajectories, with the longest simulation started from ε involving 250,000 total Monte Carlo steps.