The relative energy (∆E) of a known crystal structure is defined as the energy difference between its calculated energy and the energy of the lowest energy predicted structure that is not observed experimentally. Under the assumption that observed crystal structures always correspond to the lowest energy possible structures, and that all polymorphs of the studied molecules have been found, the reported polymorphs should correspond to the lowest energy structures within the CSP structure sets, corresponding to N lower = 0 and ∆E < 0 for all polymorphs of each molecule. Deviations from N lower = 0 and ∆E < 0 indicate either additional, kinetic, structure-determining effects or deficiencies in the model used to rank the lattice energies. To further assess the accuracy of the fragment-based energy models, we examined calculated polymorph lattice energy differences for three molecules, oxalic acid, tetrolic acid and adenine, where the polymorph stability order and, for oxalic acid and adenine, the measured energy difference is known. To extend this comparison to popular solid state DFT-D methods, periodic DFT-D calculations were carried out in the VASP software package , using the PBE exchange-correlation functional with Grimme D3 dispersion (PBE-D3).