Can the Formation of Pharmaceutical Cocrystals Be Computationally Predicted? I. Comparison of Lattice Energies

A cocrystal is only expected to form if it is thermodynamically more stable than the crystals of its components. To test whether this can be predicted with a current computational methodology, we compare the lattice energies of 12 cocrystals of 4-aminobenzoic acid, 8 of succinic acid and 6 of caffeine, with the sums of the lattice energies of their components. These three molecules were chosen for their potential use in pharmaceutical cocrystals and because they had sufficient determinations of cocrystals and corresponding partner crystal structures in the Cambridge Structural Database. The lattice energies were evaluated using anisotropic intermolecular atom-atom potentials, with the electrostatic model and the intramolecular energy penalty for changes in specified torsion angles derived from ab initio calculations on the isolated molecules. The majority of the cocrystals are calculated to be more stable than their components, but the energy difference is only large in a few of the cases where the partner molecule, cannot hydrogen bond to itself. More typically, the cocrystal stabilization is comparable to polymorphic energy differences and some of the specifically identified errors in the computational modeling. The cocrystals will be more stable relative to the observed disordered structures of caffeine and the kinetically preferred polymorph of 4-aminobenzoic acid, highlighting kinetic factors that may be involved in cocrystal formation. Overall, it appears that cocrystal formation should generally be predictable by comparing the relative stability of the most stable cocrystal and its pure components found on the computed crystal energy landscapes, but this is often very demanding of the accuracy of the method used to calculate the crystal energy.