The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed.

Computational crystal structure prediction (CSP) methods can now be applied to the smaller pharmaceutical molecules currently in drug development. We review the recent uses of computed crystal energy landscapes for pharmaceuticals, concentrating on examples where they have been used in collaboration with industrial-style experimental solid form screening. There is a strong complementarity in aiding experiment to find and characterise practically important solid forms and understanding the nature of the solid form landscape.

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A comprehensive characterization (thermal, spectroscopic, crystallographic, temperature- and moisture-dependent stability, and transition characteristics) of solvates of aripiprazole (APZ) with methanol (1:1), ethanol (2:1), dichloroethane (2:1), and a monohydrate is presented. To gain insight into packing similarities and differences, the four hydrate/solvate crystal structures and five APZ modifications were compared using the program XPac. It was found that all forms apart from the hydrate are based on either a common dimeric or catemeric motif of H-bonded APZ molecules, and this analysis confirmed also the isostructurality of the three solvates and pointed to possible mechanisms for the desolvation of the solvates and the transformation between forms X° and I. The fact that the intermolecular interactions in the monohydrate are completely different from those found in the isostructural solvates was further confirmed by analyzing the Hirshfeld fingerprint plots. The desolvation of all solvated forms results in form III. The order of their measured stabilities correlates well with variations in the intermolecular APZ/solvent interactions. Additionally, solubility and solvent-mediated transition rates were determined in a 1-PrOH/water mixture (3:7) for transformations to the monohydrate from the metastable form III and from the thermodynamically stable form at room temperature (form X°).

We report the structure of the fifth monohydrate of gallic acid and two additional anhydrate polymorphs and evidence of at least 22 other solvates formed, many containing water and another solvent. This unprecedented number of monohydrate polymorphs and diversity of solid forms is consistent with the anhydrate and monohydrate crystal energy landscapes, showing both a wide range of packing motifs and also some structures differing only in proton positions. By aiding the solution of structures from powder X-ray diffraction data and guiding the screening, the computational studies help explain the complex polymorphism of gallic acid. This is industrially relevant, as the three anhydrates are stable at ambient conditions but hydration/dehydration behavior is very dependent on relative humidity and phase purity.