Modeling Morphologies of Organic Crystals via Kinetic Monte Carlo Simulations: Noncentrosymmetric Growth Units

Crystal morphologies are governed by growth conditions, such as supersaturation and temperature, as well as by the bonding structures of growth units within the crystal lattice. The vast majority of organic crystals of interest exhibit noncentrosymmetric growth units, which feature anisotropic bonding interactions. Bonding anisotropy results in the presence of multiple distinct growth units within the unit cell that generate complex periodic bond chains and introduce edge stability phenomena, presenting a significant challenge for contemporary morphology prediction tools. Previous work has reported the use of kinetic Monte Carlo (kMC) simulations to predict the morphologies of organic centrosymmetric crystals. In this article, we consider the case of noncentrosymmetric molecules with two distinct growth units present in the unit cell (Z = 2). kMC simulations offer insight into noncentrosymmetric phenomena and provide a route to calculate relevant parameters such as step velocities and face growth rates as functions of the crystal anisotropy for different edge configurations. We employ these approaches to predict the crystal morphology of a triclinic polymorph of piracetam and discuss the extension of this approach to additional cases. We also discuss challenges inherent to modeling and simulating noncentrosymmetric crystal growth and explore rate rescaling techniques to accelerate rare event sampling and minimize computational requirements.