The energy-density distribution of predicted crystal structures of oxalic acid is presented in Fig. , showing all structures that lie within 25 kJ mol -1 of the global minimum from the force field calculations. While the two known polymorphs, α and β , have very different packing motifs, their experimentally determined lattice energies are very close, with α lying slightly lower than β . The force field lattice energies predict the β polymorph to be the lowest energy structure (Fig. ), but incorrectly produce over 70 other crystal structures below α, which is more than 7 kJ mol -1 above the global minimum.