The crystalline materials with nonlinear optical (NLO) properties are critically important for several technological applications, including nanophotonic and second harmonic generation devices. Urea is often considered to be a standard NLO material, due to the combination of non-centrosymmetric crystal packing and capacity for intramolecular charge transfer. Various approaches to crystal engineering of non-centrosymmetric molecular materials were reported in the literature. Here we propose using global lattice energy minimization to predict the crystal packing from the first principles. We developed a methodology that includes the following: (1) parameter derivation for polarizable force field AMOEBA; (2) local minimizations of crystal structures with these parameters, combined with the evolutionary algorithm for a global minimum search, implemented in program USPEX; (3) filtering out duplicate polymorphs produced; (4) reoptimization and final ranking based on density functional theory (DFT) with many-body dispersion (MBD) correction; and (5) prediction of the second-order susceptibility tensor by finite field approach. This methodology was applied to predict virtual urea polymorphs. After filtering based on packing similarity, only two distinct packing modes were predicted: one experimental and one hypothetical. DFT + MBD ranking established non-centrosymmetric crystal packing as the global minimum, in agreement with the experiment. Finite field approach was used to predict nonlinear susceptibility, and H-bonding was found to account for a 2.5-fold increase in molecular hyperpolarizability to the bulk value.

We report on the development of a novel methodology for computational predictions of the mechanical properties for single crystals. This methodology is based on constrained optimization using dispersion-corrected density functional theory level, and can be dubbed the virtual tensile test. The approach was validated on the example of 4-bromophenyl 4-bromobenzoate, an organic compound known to form three polymorphs with different mechanical characteristics. Each one of these polymorphic crystal structures was stretched stepwise along each crystallographic axis, while the remaining lattice parameters and atomic coordinates were relaxed. The geometrical properties of halogen bonds and the other noncovalent interactions were monitored at each step to understand the nature of mechanical response. The unit cell volumes and lattice energies were plotted as functions of the stretching parameter, and these curves were analyzed in terms of mechanical properties of the brittle, plastic, and elastic polymorphs.

Recently, we proposed a computational design strategy for organic nonlinear optical materials, based on the global minimization of lattice energy to predict the crystal packing from the first principles. Here, we validate this strategy on triiodobenzenes, which include CH···I hydrogen and I···I halogen bonding as the structure-determining components of their intermolecular interactions. To refine the van der Waals (vdW) parameters for an I atom, the ab initio potential surfaces for the model dimers were calculated at the CCSD­(T)/cc-pVTZ + CP theory level. The hydrogen bond C–H···I was found to have an interaction energy of −0.5 kcal/mol. The I···I contact of type I (140°–140°) was found to be attractive with a well depth of −0.4 kcal/mol at a 4.6 Å distance, whereas type II contact (180°–90°) was found to be nearly twice more attractive. Its potential well depth reaches −0.7 kcal/mol at an I···I distance of 4.4 Å. These binding energies are therefore weaker than that of the typical hydrogen bonds. The AMOEBA force-field vdW parameters were fit to describe these interactions and used to predict the crystal structures. Our structure prediction, followed by density functional theorymany-body dispersion ranking established the noncentrosymmetric crystal packing to be the global minimum, in agreement with the experimental data. The coupled perturbed Kohn–Sham approach was used to estimate nonlinear susceptibility, and the predicted values were compared to that of the urea standard. The statistical analysis of the angular distribution for the I···I contacts in the predicted virtual polymorphs was compared to that found among the experimental crystal structures of iodoaromatic compounds. In both cases, symmetric (type I) contacts dominate for shorter and longer I···I distances, whereas L-shaped (type II) contacts are preferred for intermediate distances.

Computational methods can potentially accelerate development of more efficient organic materials for second harmonic generation. Here, we test the method that includes the evolutionary algorithm for predicting crystal structure and prognosis of nonlinear optical properties based on the predicted structure. For this test, we selected 2-iodo-3hydroxypyridine, which exhibits second harmonic generation intensity comparable to that of urea. We performed global minimization of the lattice energy and found the experimental structure when many-body dispersion correction is added to the density functional theory values. We analyzed geometric preferences of the halogen bonding in predicted virtual polymorphs. We also found linear correlation between the lengths of the iodine−iodine halogen bonds and calculated second order susceptibilities.

This study continues the virtual tensile test development for molecular crystals, where polymorphs exhibit different mechanical properties. Brittle, plastic, and elastic polymorphs of 4bromophenyl 4-bromobenzoate are considered. Their properties are shown to depend on the crystal packing and topology of the hydrogen and halogen bonds. We also performed global minimization of the lattice energy to predict new polymorphs of 4-bromophenyl 4-bromobenzoate crystal structures. The most stable candidates were identified among the lowest lattice energy structures at the density functional theory level, including many-body dispersion correction. All three known polymorphic forms were successfully identified among the 60 best structures using the proposed methodology, and their diverse mechanical properties were confirmed computationally.