Average ¹²CO(1–0) excitation temperatures in different density bins in the large-scale ISM simulation. The colours correspond to the situation after a given number of iterations, as indicated in the legend. The left frames show results for a model with a of 256³ cell root grid and N_L = 3, and the right frame shows this for the same model with N_L = 4. For a given density, the grey bands indicate the 1%-99% and 10%–90% intervals of the T_ex values. The lower plots show the differences in the T_ex values between iterations 1-5 and the seventh iteration (an approximation of the fully converged solution). The horizontal dashed black line corresponds to the ΔT_ex = 0 K level.