Analytic calculations of the first H₂O peak abundance as a function of the temperature at the first H₂O peak (Appendix A, using the thermal rate coefficient from Agúndez et al. 2010; Veselinova et al. 2021). The state-specific rate coefficients are not considered in the analytic calculations. Using the state-specific chemistry enhances the abundance of H₂O by a factor 2. The dotted blue line represents the calculation of the abundance of H₂O formed by ion-neutral reactions and the dashed red line represents the calculation of the abundance of H₂O formed by neutral–neutral reactions. The triangles are the first H₂O peak abundances calculated with the Meudon PDR code for models at incident UV field and different thermal pressure.