As Fig. 4 but for A2 and A3. The panels show results for n_H = 10   cm^-3; results for other ambient densities are indistinguishable, and the accuracy of all criteria increases somewhat with increasing cell optical depth (i.e. decreasing numerical resolution). The panels show results for A2 with cell optical depth Δτ₀ ≃ 1 and timestep criteria dt00-04 (top left), dt05-dt08 (top right), and dt09-dt12 (centre left), and for A3 with dt00-dt04 and Δτ₀ ≃ 1 (centre right), Δτ₀ ≃ 3 (bottom left), and Δτ₀ ≃ 30 (bottom right). In all plots the reference result in the heavy black line is from A3 with dt12.