Ideally, one would prefer to evaluate the predicted crystal structures by the highest accuracy possible, however, even after the speed-up gained from using GTOs and exploiting the symmetry of crystals, the cost of hybrid functional calculations is still prohibitively high. In the specific case of 526 single point energy evaluation of the oxalic acid data set, calculations at force field, PBE and PBE0 levels of theory using 40 2.0 GHz Intel Skylake processors, took under a minute, 11 hours, and 101 days, respec- tively. The corresponding cost ratio is roughly 1:1,000:220,000, and is expected to be more extreme for larger molecules. This large ratio of computational costs highlights the motivation for an efficient, reliable estimation of PBE0 lattice energies. Average timings for each molecule at each level of theory are provided in the SI (Table ).