Indigo (Fig. ) is known to form two polymorphs, named A and B, both containing layers of hydrogen bonded molecules. The structure of these layers is almost unchanged between polymorphs A and B, but their structures differ in the arrangement of these layers (see overlay, Fig. ). Thus, the lowest energy pathway between polymorphs should not disrupt the hydrogen bonding and is expected to involve a relatively low energy barrier. Both polymorphs have space group symmetry P 2 1 /c with half a molecule in the asymmetric unit of the unit cell (both molecules lying on centres of inversion) and thus two molecules in the unit cell. To allow free molecular translations, unconstrained by the position of crystallographic inversion centres, the most symmetric representation used for simulation of these structures was P 2 1 with whole molecules in the asymmetric unit. Crystal structure prediction in P 2 1 finds polymorphs A and B as the two lowest energy crystal structures (see Fig. ), with B having a calculated lattice energy 4.0 kJ mol -1 below A. For comparison, the two polymorphs are reported to be nearly equi-energetic when evaluated using periodic DFT with a plane wave basis set and many-body dispersion correction. Monte Carlo simulations were started from the CSP structures matching both polymorphs, which were continued for 10 lid energies, incremented by 5.0 kJ mol -1 with 1,000 attempted steps under each lid (covering a total 50.0 kJ/mol energy window). From threshold simulations in space group P 2 1 , the first connection between polymorphs A and B is found when the lid is 26.0 kJ mol -1 above A (30.0 kJ mol -1 above B) (Fig. ). Below this energy, no other structures are connected to B, one slightly higher energy structure is connected to A and two additional structures connect to A and B at the same lid energy. All three of the additional crystal structures within the basin connected to A and B maintain the same hydrogen bond motif, but with greater differences in molecular orientation around the hydrogen bonds than between polymorphs A and B. No further structures are connected to the basin containing these five local energy minima (A, B and the three higher energy structures) until the lid energy is raised a further 15 kJ mol -1 . Globally, the results give a picture of an energy landscape that is funneled towards a small set of structures, all featuring the same favoured hydrogen bonding motif, with the two known polymorphs as the lowest energy local minima within this energy superbasin.