Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals, as most drugs are marketed in the form of crystals of the pharmaceutically active ingredient. Molecular crystals are bound by dispersion (van der Waals) interactions, whose weak nature generates potential energy landscapes with many shallow minima that are close in energy. As a result, molecular crystals often exhibit polymorphism, the ability of the same molecule to crystallize in several structures. Crystal structure may profoundly influence the bioavailability, toxicity, manufacturability, and stability of drugs. In the context of technological applications, crystal structure affects the electronic and optical properties.

We perform large scale quantum mechanical simulations to predict the structure of molecular crystals and investigate the effect of crystal packing on their electronic and optical properties. The massively parallel genetic algorithm (GA) package, GAtor, relies on the evolutionary principle of survival of the fittest to find low-energy crystal structures of a given molecule. Dispersion-inclusive density functional theory (DFT) is used for structural relaxation and accurate energy evaluations [1]. The structure generation package, Genarris, performs fast screening of randomly generated structures with a Harris approximation, whereby the molecular crystal density is constructed by replicating the single molecule density, which is calculated only once. Many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation (BSE), is then employed to describe properties derived from charged and neutral excitations.

For tricyano-1,4-dithiino[c]-isothiazole (TCS3), we propose a layered crystal structure, which is only 20 meV higher in energy than the observed cyclic dimer structure, and may exhibit a smaller gap, better transport properties, and broader optical absorption [2]. For rubrene, we predict that a lesser known monoclinic crystal structure may exhibit higher singlet fission efficiency than the orthorhombic form, possibly rivaling that of pentacene [3]. Thus, the electronic and optical properties of organic semiconductors may by optimized by modifying the crystal packing.

[1] N. Marom, R. A. DiStasio, Jr. , V. Atalla, S. Levchenko, A.M. Reilly, J. R. Chelikowsky, L. Leiserowitz, A. Tkatchenko, Angew. Chem. Int. Ed. 125, 6761 (2013)

[2]  F. S. Curtis, X. Wang, and N. Marom, Acta Cryst. B 72, 562 (2016)

      [3] X. Wang, T. Garcia, S. Monaco, B. Schatschneider, N. Marom, CrystEngComm 18, 7353 (2016)