Phase diagrams. Left panel: Phase diagram (pressure, P, vs. gas density, n_H) of case A. The WNM and CNM regions are labeled. Right panel: Dependence of the ionization fraction, X_e, obtained with Eq. (6), vs the gas temperature, T. Here, the CR ionization rate is set to the value of ζ^H (see Sect. 2.4). Colors in both panels correspond to the log₁₀ of the density of points as shown by the same color bar on the right. The gray part of the right-panel plot shows the voxels that were artificially set to X_e = 1 (see the two black arrows) as they strongly depart from the assumptions that justify the use of Eq. (6). See main text for details.