To investigate this point, we next applied threshold clustering to the CSP landscape of acrylic acid, which features a number of hydrogen bonding motifs, including dimers, chains, and even tetramers, as well as matches to both of the experimental forms, I and II. The results of the threshold simulations on the 100 lowest energy structures, which represent an energy cutoff of 5.0 kJ mol -1 from the global energy minimum, with an energy lid of 5.0 kJ mol -1 is shown in Figure . The disconnectivity graph reveals a larger number of basins than seen for benzene, consistent with the stronger interactions leading to higher energy barriers, and moreover, by colouring the structures that correspond to either dimer or chain hydrogen bonding motifs, it is apparent the basins are largely distinguished by these motifs. However, there are notable exceptions in a number of the dimer basins where mixed dimer-chain structures are observed (Fig. ). In one basin, we even identify pathways from dimer structures to chain structures. The mixed structures are particularly interesting as they present a situation in which half of the unit cell has shifted from the dimer motif to the chain motif, a possible intermediate stage between chain and dimer structures. These transitions between hydrogen-bonded chain and dimer structures are only observed in trajectories initiated from structures where the carboxylic acid groups are arranged in columns, each group directly above and below another (Fig. ). No transitions are observed for dimer structures where the carboxylic acid groups are surrounded by alkene groups, which appears to inhibit facile inter-conversion.