Average fractional abundances of ${\text{H}}_3^{+}$, its deuterated isotopo-logues, and electrons in the core model as functions of the cosmic-ray ionisation rate, ${\zeta }_{{\text{H}}_2}$. The solid red curve shows the total (ortho + para) abundance of H₂D⁺, whereas the dashed red curve shows the fractional oH₂D⁺ abundance separately. Likewise, the dashed green curve shows the fractional pD₂H⁺ abundance. The combined fractional abundance of all modifications of ${\text{H}}_3^{+}$, denoted $\widetilde{{\text{H}}_3^{+}}$ in the text, is shown with the dashed cyan curve.