The choice of the calculation method for collecting the high level PBE0 data points is more significant. This is because of the extremely high cost of exact exchange calculations when employing delocalized basis sets such as PWs, which has limited the use of such sets in hybrid functional calculations. GTOs, on the other hand, require much less (up to 2 orders of magnitude) resources to achieve equivalent accuracy of a PW method. The cost of GTO calculations depends on the size of basis set and basis sets of at least triple-zeta quality including one set of polarization functions have been shown to provide good agreement with converged PW results. Thus, we have performed a comparison of GTO basis sets to PW calculations on the lattice energies of one of our sets of CSP structures, oxalic acid. We first compared the results of well-converged VASP PBE lattice energies on the full set of 526 oxalic acid crystal structures to those from CRYSTAL17 using the Ahlrich's-type split valence double-zeta (def2-SVP) and triple-zeta (def2-TZVP) ba-sis sets (Fig ). Enlarging the basis set from def2-SVP (Fig. ) to def2-TZVP (Fig. ) reduces the mean absolute error (MAE) and maximum absolute error (MAX) in relative lattice energies (always measured relative to the global energy minimum structure, which is the same in PBE and PBE0), and improves the correlation to PW results considerably. Absolute lattice energies are also improved significantly, from an MAE of 43 kJ mol -1 with def2-SVP to less than 1 kJ mol -1 with def2-TZVP (Fig. ). The PBE/def2-TZVP results show an excellent linear relationship to those from PW calculations, as well as small differences in the absolute lattice energies.