Elastic Tensors from Pairwise Energy Frameworks in Molecular Crystals

The mechanical properties of molecular crystals are fundamentally important in their industrial applications across pharmaceuticals, agrochemicals, energetic materials and other areas. Despite this, complete measurement or even computational prediction of elastic tensors for molecular crystals is anything but commonplace. The absence of rapid, reliable and broadly applicable methods in this endeavor frequently leads chemists to rely on intuitive ideas and examination of pairwise intermolecular interactions such as hydrogen- or halogen-bonds in order to rationalize the mechanical behavior of molecular crystals. Such perspectives are widespread in contemporary literature, but the extent to which these notions yield reliable and quantifiable insight is itself relatively unexplored. We propose a simple approximation, the Equilibrium Pairwise Model (EPM), compatible with any method to predict intermolecular interaction energies, that directly and efficiently yields an estimate of the complete elastic tensor. The protocol can be performed for any given molecular crystal structure, even those directly from experiment (i.e., without geometry optimization), and is guaranteed by construction to yield a positive-definite result─in contrast to conventional methods where computing valid elastic tensors for molecular crystals can prove challenging even for well-established and otherwise accurate model chemistries. We examine the accuracy of this protocol, along with other classical and contemporary methods, against experiment and periodic (plane-wave) density functional theory calculations to assess their reliability and accuracy. Through examination of the failures and successes, we aim to provide chemical insight into the kinds of materials where the model and, more broadly, thinking based on pairwise intermolecular interactions can reliably explain mechanical or other material properties and where they should be avoided.