Predicting Organic Crystal Lattice Energies with Chemical Accuracy

A fast, fragment-based hybrid many-body interaction model is used to optimize the structures of five small-molecule organic crystals (with fixed experimental lattice parameters) and predict their lattice energies with accuracies of 2-4 kJ/mol compared to experiment. This model treats individual molecules in the central unit cell and their short-range two-body interactions quantum mechanically, while long-range electrostatics and many-body induction are treated with a classical polarizable force field. For the hydrogen bonded ice, formamide, and acetamide crystals, MP2 calculations extrapolated to the complete-basis-set limit provide good accuracy. However, MP2 exhibits difficulties for crystals such as benzene and imidazole, where pi-stacking dispersion interactions are important, and post-MP2 corrections determined from small-basis-set CCSD(T) calculations are required to achieve chemical accuracy. Using these techniques, accurate crystal lattice energy predictions for small-molecule organic crystals are feasible with currently available computing power.