Molecular Crystals

Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals because most drugs are marketed as crystals of the pharmaceutically active ingredient. Molecular crystals are held together by dispersion (van der Waals) interactions. These weak but long-ranged forces arise from electrostatic interactions between multipole moments generated by quantum mechanical fluctuations in the electron density. The weak nature of dispersion interactions produces potential energy landscapes with many shallow local minima. This is why molecular crystals often exhibit polymorphism, i.e. the same molecule may crystallize in several different structures, which may possess markedly different physical and chemical properties. A highly accurate fully quantum mechanical approach is needed to capture extremely small energy differences between molecular crystal polymorphs. To predict the structure of molecular crystals we use dispersion-inclusive density functional theory (DFT) coupled with a genetic algorithm.

The best 100 putative structures of tricyano-1,4-dithiino[c]-isothiazole (TCS3), generated by the GAtor genetic algorithm, ranked with different dispersion-inclusive DFT methods. Different DFT functionals and dispersion methods systematically favor specific packing motifs. The Tkatchenko-Scheffler (TS) pairwise method systematically destabilizes catemer-like packing motifs, while the PBE semi-local functional systematically stabilizes layered structures. Only the PBE0 hybrid functional combined with the many-body dispersion (MBD) method ranks the experimentally observed structure as the most stable. 

Acta Cryst. B 72, 562 (2016)

The potential energy surface in the a-b plane of the γ polymorph of glycine, calculated with the semi-local PBE functional combined with the Tkatchenko-Scheffler (TS) pairwise dispersion method and the many-body dispersion (MBD) method. The MBD method yields the correct lattice vectors by accounting for long-range screening and for higher order non-pairwise-additive dispersion interactions. In addition, the MBD method provides the meV accuracy needed to correctly describe the relative energies of the polymorphs of glycine and other molecular crystals. Angew. Chem. Int. Ed. 52, 6629 (2013)

Van der Waals interactions:

Gecko on the wall

thinks of flies but not at all

of dipole-dipole