between ice and water appears crucial for a reasonable isobar shape. Functionals that overestimate the density dierence completely fail to generate a density maximum, instead leading to an abrupt transition to a low-density amorphous ice phase (e.g. optPBE). Conversely, functionals with a small density dierence or one in the correct range typically yield more reasonable isobar results, albeit potentially shifted in temperature and density. Notably, the SCAN functional falls into this latter category, with a density that is too high (5%) and a temperature deviation of almost 20%.