Molecular Crystals: A Test System for Weak Bonding

Molecular crystals are classified by the leading order power law for the long-range interaction among the molecular constituents and additionally by hydrogen bonds. It is found that for molecular crystals formally dominated by ionic, electrostatic dipole−dipole or quadrupole−quadrupole, and even van der Waals interactions, reasonably accurate predictions of crystal structure can be made. The PBE density functional approximation leads to a systematic overestimation of lattice parameters of type length reaching 7% on average for the van der Waals class. This is consistent with the findings of earlier investigations. The PBEsol functional has a signed mean deviation of <1% for normal crystals and molecular crystals up to the dipole class. For the highest class the mean deviation remains below 2%. The scatter around the mean deviation reaches ≈3% rms. The basic Kohn−Sham local density approximation remains below 2% signed mean deviation across all classes. The scatter around the mean deviation for molecular compounds remains below 2%. Thus, certain density functionals, such as PBEsol and KS, yield about equally accurate predictions for molecular crystal parameters as do well performing functionals for strongly bound solids.