Dispersion XDM with Hybrid Functionals: Delocalization Error and Halogen Bonding in Molecular Crystals

The accurate calculation of relative lattice energies of molecular crystals is important in polymorph ranking and crystal structure prediction. Delocalization error has been shown to affect calculated intermolecular binding energies in DFT and is similarly expected to affect the lattice energies of some classes of molecular crystals. In this work, we explore the use of dispersion-corrected hybrid functionals in the planewave-pseudopotentials approach to reduce delocalization error. We combine several hybrid functionals with the exchange-hole dipole moment (XDM) model for dispersion and show that they generally outperform GGA functionals in the calculation of both gas-phase binding energies and molecular crystal lattice energies. We apply the resulting XDM-corrected functionals to four halogen-bonded crystals: Cl₂, Br₂, I₂, and ICl. GGA functionals severely overestimate their lattice energies, while hybrid functionals give accurate values. The preference of GGA functionals for monatomic structures in the Br₂ and Cl₂ crystals is also explained. Finally, we apply a recently developed method to calculate Bader's delocalization indices to examine the extent of intermolecular delocalization in the halogen molecular crystals. It is shown that intermolecular delocalization indices can be used to measure the strength of halogen bonds within the crystal, as well as detect the presence of delocalization error.