The rank correlation of crystal structures between the FIT+DMA+GP and FIT+DMA +MP2 models is only slightly lower than for oxalic acid (τ = 0.853 for the GP model trained with 10% of dimers, τ = 0.943 at 20%, compared to τ = 0.501 comparing FIT+DMA to FIT+DMA+MP2). This demonstrates that the ranking of most predicted structures is correct at the lowest training fraction. However, it is clear (see Fig. ) that, using the GP model trained with 10% of dimers, the relative energies of the known polymorphs is incorrect. This may be because almost all of the dimer energy corrections for the three observed polymorphs have been predicted by the GP model (i.e. are not in the training set). In particular, the stability of the triclinic polymorph, which is least stable of the three observed forms using FIT+DMA+MP2, is exaggerated. For this structure, only 2% of dimers are in the 10% training set, so errors related to the GP model will be higher for this structure than the average. Comparatively, in the oxalic acid 10% landscape (Fig. ), both known polymorphs have a smaller fractions of 87% and 90% of the dimers predicted by the FIT+DMA+GP model for α and β, respectively.