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

We consider several molecules characterised by pi-electron conjugation whose extent changes along a flexible torsional coordinate, and which represent the monomer units of polymorphic molecular crystals. Delocalisation error in density functional theory (DFT) adversely impacts conformational energies in these species, overstabilising the conformation that maximises 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.

Automated summary

This study considers molecules with pi-electron conjugation and finds that delocalisation error in density functional theory affects their conformational energies. By using density-corrected methods, the errors can be significantly reduced, enabling accurate calculations of molecular crystals.