Correcting pi-delocalization errors in conformational energies using density-corrected DFT, with application to crystal polymorphs

We consider several molecules characterized by pi-electron conjugation whose extent changes along a flexible torsional coordinate, and which represent the monomer units of polymorphic molecular crystals. Delocalization error in density functional theory (DFT) adversely impacts conformational energies in these species, overstabilizing the conformation that maximizes conjugation length and leading to incorrect relative energies for the corresponding crystal polymorphs. We demonstrate that density-corrected (DC-)DFT, in which a DFT exchange-correlation functional is evaluated using a Hartree-Fock density, significantly reduces these conformational energy errors. When DC-DFT monomer energies are used as a low-cost intramolecular correction to a periodic DFT calculation of the molecular crystal, the resulting relative polymorph energies are within 1 kJ/mol of benchmark results.