﹩N({{\rm{H}}}_{3}^{+})/N({{\rm{H}}}_{2})﹩ column density ratios predicted for diffuse and translucent molecular clouds exposed to UV radiation with ﹩{\chi }_{\mathrm{UV}}/{n}_{250}=1﹩. Results are shown for several values of the total visual extinction through the cloud. The blue dashed line indicates results obtained using the analytic treatment adopted by IM12 (see our Equation (4)), for an assumed gas temperature of 70 K.