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Liquid water’s isothermal compressibility and isobaric heat capacity, and the magnitude of its thermal expansion coefficient, increase sharply on cooling below the equilibrium freezing point. Many experimental^,,,,, theoretical^,, and computational^, studies have sought to understand the molecular origin and implications of this anomalous behaviour. Of the different theoretical scenarios^,, put forward, one posits the existence of a first-order phase transition that involves two forms of liquid water and terminates at a critical point located at deeply supercooled conditions^,. Some experimental evidence is consistent with this hypothesis^,, but no definitive proof of a liquid–liquid transition in water has been obtained to date: rapid ice crystallization has so far prevented decisive measurements on deeply supercooled water, although this challenge has been overcome recently. Computer simulations are therefore crucial for …