We refer the reader to Refs. 21 and 22 for comprehensive reviews of the mechanical properties of MOFs and their applications. We note, in particular, many soft porous crystals are also "soft" in the mechanical sense of the word: they often have strong anisotropy in their elastic properties, and exhibit directions of low Young's and shear modulus. Many framework materials display mechanical behavior not commonly found in dense crystalline materials, among which negative linear compressibility and auxeticity (negative Poisson's ratio). While anisotropic properties can be difficult to access experimentally, their determination has become relatively routine in atomistic calculations, whether at the level of Density Functional Theory (DFT) or through molecular dynamics (MD) based on a classical force field. Both methods allow the determination of the full second-order elastic tensor, and through tensorial analysis the calculation of the various directional moduli and their polycrystalline averages.