Abstract

The physical properties of supercooled water have been a source of continued interest since the pioneering work of Speedy and Angell, who reported sharp increases in the response functions upon isobaric cooling [1]. One intriguing hypothesis that has been formulated to explain this behavior is the existence of a metastable liquid-liquid transition at deeply supercooled conditions [2]. The preponderance of experimental evidence is consistent with this hypothesis (e.g., [3], [4]), although no definitive proof exists to date. Computational studies have played an important role in this area [2], [5]-[13]. State-of-the-art free energy techniques provide clear evidence of a liquid-liquid transition in the ST2 model [14] of water [15], including the identification of three phases at the same, deeply supercooled thermodynamic conditions: two metastable liquids in equilibrium, and a stable crystal [15]. Recent calculations on tunable tetrahedral models support this key conclusion of the free energy results [16], [17]. A necessary condition for the existence of a phase transition between two supercooled phases is a wide separation of time scales between nucleation and structural relaxation. Understanding what aspects of intermolecular force fields give rise to this separation of time scales is an important open question.

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