Elastic Tensors from Pairwise Energy Frameworks in Molecular Crystals

Mechanical properties of molecular crystals are fundamentally important to their in- dustrial utility as pharmaceuticals or agrochemicals, energetic materials and more. Yet, complete measurements of elastic tensors are relatively rare and even their theoretical prediction via conventional energy models is not commonplace for molecular crystals. The absence of reliable, widely applicable methods in this area leads chemists to rely on the intuitive and foundational notion that we can rationalise the mechanical behaviour of molecular crystals by examination of pairwise intermolecular interactions. This view is pervasive in contemporary literature, but the extent to which this idea yields reliable and quantifiable insight is itself relatively unexplored. We propose a simple approxima- tion, compatible with any intermolecular energy model, to rapidly produce an estimate of the complete elastic tensor, given an experimental crystal structure. The protocol can be performed without geometry optimisation and is guaranteed by construction to yield a positive-definite tensor. We benchmark our technique, along with other classical and contemporary methods, against experiment and periodic (plane-wave) density functional theory calculations in order to assess where the approximation is useful. Further, by examining the failure cases we hope to provide chemical insight into classes of materials where we should not rely on intuitive explanations based on pairwise intermolecular interactions to explain mechanical or other material properties.