Accurate Molecular Crystal Lattice Energies from a Fragment QM/MM Approach with On-the-Fly Ab Initio Force Field...

We combine quantum and classical mechanics in a fragment-based many-body interaction model to predict organic molecular crystal lattice energies. Individual molecules in the central unit cell and their short-range pairwise interactions are modeled quantum mechanically, while long-range pairwise and many-body interactions are approximated classically. The classical contributions are evaluated using an accurate ab initio force field that is constructed on-the-fly from quantum mechanical calculations on the individual molecules in the unit cell. The force field parameters include ab initio distributed multipole moments, distributed polarizabilities, and isotropic two- and three-body atomic dispersion coefficients. This QM/MM fragment model reproduces full periodic MP2 lattice energies to within a couple kJ/mol at substantially reduced cost. When high-level electronic structure methods are coupled with the ab initio force field, molecular crystal lattice energies are predicted to within 2 kJ/mol of their experimental values for six of the seven crystals examined here. Finally, Axilrod-Teller-Muto three-body dispersion energy plays a nontrivial role in several of the molecular crystals studied here.