Three small molecules were chosen to test the methods developed here: oxalic acid, maleic hydrazide and urazole (Fig. ). All three are small enough so that it is affordable to evaluate the energies for all predicted crystal structures at the highest level of theory considered here, so that errors from the GPR models can be assessed. Two of the molecules (oxalic acid and maleic hydrazide) have known polymorphism and all three are known to be challenging for obtaining accurate energy rankings either between their known polymorphs or in previous CSP studies. (a) oxalic acid (b) maleic hydrazide (c) urazole The data sets used here consist of low-energy crystal structures found in previous CSP studies. The crystal structures were generated using quasi-random sampling followed by local energy minimization to locate the local minima on each molecule's lattice energy hypersurface using the Global Lattice Energy Explorer code. Crystal structures were generated in the 11 most frequently occupied space groups (P2 1 /c, P2 1 2 1 2 1 , P1, P2 1 , Pbca, C2/c, Pna2 1 , Cc, Pca2 1 , C2, P1) with one independent molecule in the crystallographic asymmetric unit (Z = 1). The molecular structure was held rigid throughout, at the DFT optimized geometry of the isolated molecule.