Crystal packing plays an important but difficult-topredict role in controlling physical properties of organic molecular crystals, including solubility, shelf stability, and bioavailability. Polymorphism, or the potential to form multiple distinct crystal packing motifs, is critically important to the pharmaceutical industry, as highlighted by the case of the HIV drug ritonavir, where the appearance of a previously-unknown and lowsolubility polymorph made it impossible to produce the original polymorph and forced a drug recall. Other pharmaceutical examples of such "disappearing polymorphs" are known, and more recently it has been suggested that the thermodynamically stable crystal form has not been found experimentally for 15-45% small-molecule drugs currently available on the market. Over the past decade, ab initio crystal structure prediction has moved from the category of "impossible", as it was famously characterized 30 years ago, to merely "extremely challenging". Successful crystal structure prediction, including quantitative ranking of low-energy polymorphs, is most feasible for small-molecule species with limited conformational flexibility. Quantitative prediction of thermodynamically stable polymorphs comes with stringent accuracy requirements, as an estimated 50% of polymorph pairs differ by less than 2 kJ/mol and around 95% of the conformers differ by less than 8 kJ/mol. Dispersion-corrected density functional theory (DFT+D) had made enormous strides towards making crystal structures predictable from first principles, but this success is mitigated in certain cases by overstabilization of delocalized π-electron systems, which is a consequence of self-interaction error (SIE). This issue is thought to impact polymorph prediction in well-known examples such as the anti-cancer drug axitnib, the organic semiconductor molecule rubrene, the well-studied ROY molecule, molecule X from the third blind test of crystal structure prediction, as well as other systems. Structures for several of these molecules are shown in Fig. . Each of these molecules exhibits conformational polymorphism whereby changes in intramolecular conformation access different intermolecular crystal packing motifs. Notably, in each of these molecules the extent of π-electron delocalization changes as a function of one or more intramolecular torsional coordinates. To the extent that delocalization error may overstabilize the more highly-conjugated conformations, we expect that DFT will produce erroneous conformational energy profiles for these flexible monomers. Those errors propagate into errors in the relative energies for the crystal polymorphs formed from these monomers. Given the small energy differences between polymorphs, even modest conformational energy errors can have a qualitative impact on polymorph stability ordering.