The measured relative stabilities of the oxalic acid polymorphs show that α is the more stable form. After correction of the measured enthalpy difference for lattice vibrational contributions, the measured lattice energy difference is approximately 0.2 kJ/mol. . The correct stability order is only predicted correctly by the FIT+DMA+MP2 model, which predicts the α form to be 0.02 kJ/mol more stable than β. This model also gives good agreement with fully relaxed solid state DFT models (Fig. ), despite only being a single point correction at the FIT+DMA geometry. To investigate the influence of the modest basis set size (6-31+G**) used in our calculations, the energies of the two oxalic acid polymorphs were also calculated using our fragment-corrected model with MP2 dimer interaction energies evaluated at the MP2/6-311++G** level of theory (shown in Fig. ). The larger basis set was found to change the relative lattice energies slightly, increasing the lattice energy difference to 0.31 kJ/mol, now in excellent agreement with experiment. For tetrolic acid, the β polymorph is known to be the more stable form below 56 -58 • C, but we are not aware of a measured energy difference between the polymorphs. The FIT+DMA force field gives the incorrect stability order, with β calculated to be 0.29 kJ/mol less stable than α. In this case, all of the fragment-corrected models correct the stability order (Fig. ), predicting β to be more stable than α by between 0.82 kJ/mol (FIT+DMA+PBE) and 1.84 kJ/mol (FIT+DMA+MP2). These results are in good agreement with the relative energy calculated using the popular solid state PBE-D3 method, which calculates the α form to be 0.93 kJ/mol less stable than β, but much smaller than the energy difference calculated with PBEh-MBD . Calculations for FIT+DMA+MP2 were again repeated with a larger basis set for the MP2 dimer interaction energies, where only minor changes in energy were observed. ). To our knowledge, an experimental value of the lattice energy difference is not available, but the β form is known to be more stable than α. Lattice energies calculated for the α form (blue) are all given relative to the β form (red). All corrections to FIT+DMA are single-point corrections at the FIT+DMA geometry, whereas PBE-D3 refers to full re-optimization from the FIT+DMA geometry.