(Top) the model I_UV is plotted as a function of inferred f_H2. (Bottom) alternatively, the molecular hydrogen fraction can be plotted as a function of the ratio of the hydrogen density of the UV radiation field strength, for our general value of n_H = 300.0 cm^-3. As expected from Black & van Dishoeck (1987), log(f_H2) is directly proportional to log(n_H/I_UV), where the intercept of this relation depends on the total H₂ column density (see also Fig. 2 in Lee et al. 2007). Ranges for inferred N(H₂) are indicated by different symbols. The horizontal arrow gives the change in log(n_H/I_UV) for an increase or decrease of n_H by a factor of 2. The vertical arrows on the data points indicate the correction of the inferred fraction to the directly observed fraction. Note that the inferred molecular fraction, f_H2, directly depends (non-linearly) on the ratio of W(CH) over W(5780), and the I_UV depends also on CH, as well as CN and E_(B−V).