Photoionisation equilibrium calculation for the temperature and ionisation state of a static H II region for a H number density n_H = 10 cm⁻³: (a) gas temperature vs. radius for 3 different calculations: dotted-red line obtained with CLOUDY, dashed-red line CLOUDY with the Case-B setting and solid-blue line shows the result with PION using the On-the-Spot approximation (see text for details); (b) ionisation state of H and He as a function of radius: solid lines are results obtained with PION, dotted lines with CLOUDY and dashed lines CLOUDY+CaseB; and (c) as (b) but showing ionisation state of O as a function of radius.