At high temperature, , the assumption of non-relativistic electron velocities becomes invalid, and a variety of physical processes such as electron-positron pair production may come into play. At very low temperatures, , many of the analytic fitting formulae used to parameterize atomic rates are unreliable, and the neglect of molecule formation and atomic fine structure cooling renders rates inaccurate. The code does not allow temperatures less than 3000K for this reason and because serious numerical errors can occur. If lower values are attempted the temperature is artificially reset to 3000K. On the high temperatures end, there is no mechanism preventing values greater than , but the user should be aware that in this regime the rates are probably unreliable.