Reliable crystal structure predictions from first principles.

An inexpensive and reliable method for molecular crystal structure predictions (CSPs) has been developed. The new CSP protocol starts from a two-dimensional graph of crystal's monomer(s) and utilizes no experimental information. Using results of quantum mechanical calculations for molecular dimers, an accurate two-body, rigid-monomer ab initio-based force field (aiFF) for the crystal is developed. Since CSPs with aiFFs are essentially as expensive as with empirical FFs, tens of thousands of plausible polymorphs generated by the crystal packing procedures can be optimized. Here we show the robustness of this protocol which found the experimental crystal within the 20 most stable predicted polymorphs for each of the 15 investigated molecules. The ranking was further refined by performing periodic density-functional theory (DFT) plus dispersion correction (pDFT+D) calculations for these 20 top-ranked polymorphs, resulting in the experimental crystal ranked as number one for all the systems studied (and the second polymorph, if known, ranked in the top few). Alternatively, the polymorphs generated can be used to improve aiFFs, which also leads to rank one predictions. The proposed CSP protocol should result in aiFFs replacing empirical FFs in CSP research.

Factors influencing hydrogen peroxide versus water inclusion in molecular crystals.

Hydrate formation is often unavoidable during crystallization, leading to performance degradation of pharmaceuticals and energetics. In some cases, water molecules trapped within crystal lattices can be substituted for hydrogen peroxide, improving the solubility of drugs and detonation performance of explosives. The present work compares hydrates and hydrogen peroxide solvates in two ways: (1) analyzing structural motifs present in crystal structures accessed from the Cambridge Structural Database and (2) developing potential energy surfaces for water and hydrogen peroxide interacting with functional groups of interest at geometries relevant to the solid state. By elucidating fundamental differences in local interactions that can be formed with molecules of hydrogen peroxide and/or water, the analyses presented here provide a foundation for the design and selection of candidate molecules for the formation of hydrogen peroxide solvates.