Average HCO⁺(1–0) excitation temperature as a function of density in the large-scale ISM simulation (cf. Fig. 4). The left and right frames show the results for different spatial discretisations (N_L = 3, 4). The grey bands correspond to the 1%–99% and 10%–90% T_ex intervals. The legend shows the colours that correspond to the different number of iterations. The lower frames show the T_ex errors over iterations 1–7, estimated relative to the final values after 12 iterations. The horizontal dashed black line shows the ΔT_ex = 0 K level.