Therefore, NTE is of great consequence to the mechanical stability of these often fragile systems. Although MOF-399 has the potential to demonstrate record-breaking porosity, this has yet to be reported experimentally, suggesting mechanical or chemical fragility. Previously, we employed density functional theory calculations to determine the maximum eigenvalue of the stiffness matrix of this material, which was found to be 6.33 GPa. However, the finite temperature calculations shown here demon-strate that at room temperature this material will be significantly less mechanically robust than first thought. We note that certain MOFs can be more resilient to temperature, such as PCN-68 which shows little variation in mechanical properties over the temperature range.