We report a multistage lattice energy minimization methodology for generating stable packing arrangements of cocrystals containing flexible molecules. In the first approximation, the intermolecular electrostatic interactions are modeled with atomic charges and the molecular deformation energy is interpolated over a set of precomputed quantum mechanical values. At subsequent stages, the accuracy is improved by first using analytically rotated and then conformation-dependent multipole moments, computed from the isolated-molecule charge density, and "on-the-fly" quantum mechanical calculations to compute the intramolecular deformation energy. This multistage approach increases the efficiency of the search and establishes the molecule-dependent error due to the atomic charge representation of the charge density and the neglect of the conformational dependence of atomic multipole moments. The methodology is used to study the lattice energy landscapes of the cocrystals of 4-aminobenzoic acid with 2,2'-bipyridine and 4-nitrophenylacetic acid, as well as the single-component crystals. All single-component, experimentally determined crystal structures within the scope of the search were found at, or very close to, the global minimum. The experimental cocrystal with 2,2'-bipyridine is also predicted to be among the most stable packing arrangements. On the contrary, the lattice energy landscape of the cocrystal with 4-nitrophenylacetic acid contains several low energy structures that are more stable than the experimentally observed form and have different hydrogen bonding motifs. Overall, the methodology can provide worthwhile crystal energy landscapes for multicomponent organic solids and thereby contribute to understanding cocrystal formation.

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.

No abstract