Gas energy densities, as a function of time, of a gas that is relaxing to radiative equilibrium from two sets of initial conditions. Marked with a blue “o” is the relaxation where the initial gas energy is set as 10² times the expected equilibrium value. Marked with a red “x” is the initial gas energy is set as 10⁻⁶ times the equilibrium value. The dashed blue and red lines represent the same heating-cooling problem, but solved with a smaller numerical time step (see text). Since the time axis is logarithmic, the initial conditions at t = 0 are not visible.