cij: A Python code for quasiharmonic thermoelasticity.

The Wu-Wenzcovitch semi-analytical method (SAM) to compute the thermoelastic tensor (Cij) (SAM-Cij) is a concise and predictive formalism to calculate the high-pressure and high-temperature (high-PT) Cij of crystalline materials. This method has been successfully applied to materials across different crystal systems in conjunction with ab initio calculations to compute static elastic coefficients and phonon frequencies. Such results have offered first-hand insights into the composition and structure of the Earth's mantle. Here we introduce the cij package, a Python implementation of the SAM-Cij formalism. It enables a thermoelasticity calculation to be initiated from a single command and fully configurable from a calculation settings file to work with solids within any crystalline system. These features allow SAM-Cij calculations to work on a personal computer and to be easily integrated as a part of high-throughput workflows. Here we show the code's performance for three minerals from different crystal systems at their relevant PTs: diopside (monoclinic), akimotoite (trigonal), and bridgmanite (orthorhombic).