Determination of water equilibrium abundance. Solid lines and dashed lines denote adiabats and demixing curves, respectively, the latter being obtained from the V23 conv-1800 K phase diagram shown in Fig. 4. We start with an overlap region where T_ad < T_dmx and an initial water abundance of Z = 0.7 throughout the entire planet. Then, Z is decreased until the overlap region disappears and T_ad ≥ T_dmx at all pressures. Here, the equilibrium abundance found is 0.67. The orange adiabat is the profile with a 0.67 outer-envelope abundance. The obtained abundance is high in this example because T_1bar = 240 K, implying a rather warm adiabat in a young planet. The demixing region for the case with Z_atm = 0.7 is much larger compared to that typically encountered in our simulations. This choice was made for visual clarity (see text for more details).