We also want to be able to analyse energy landscapes involving crystal structures with different symmetries. Here, we look at two examples involving known crystal structures with different space group symmetry. To remove the symmetry constraints, we use P 1 unit cells. In the following examples, we construct P 1 unit cells containing sufficient molecules to be able to represent all the known polymorph crystal structures, i.e., the lowest common multiple of Z (the number of formula units in the unit cell) for all known structures. Thus, the simulations presented below are not fully unconstrained -translational symmetry is imposed by the unit cell -but the models have sufficient flexibility to describe the polymorphs of interest. The approach could be extended to be able to visit more possible packing symmetries by expanding the unit cell to contain more molecules. Tetrolic acid has two known polymorphic forms: a triclinic structure in space group P 1 and a monoclinic structure in space group P 2 1 , referred to as α and β, respectively. The two structures have different hydrogen bond motifs: the carboxylic acid groups form cyclic, doubly-hydrogen bonded dimers in α and infinite hydrogen bond chains in β (Fig. ). Both crystal structures have two molecules in the unit cell, so simulations were performed using P 1 unit cells containing two molecules. The number of degrees of freedom to sample this system was the same as the nicotinamide:benzoic acid co-crystal, so we applied the same number of Monte Carlo steps (3,000) at each lid energy, which was raised in 5 kJ mol -1 increments, starting simulations from the structures of α and β.