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The hypothesis that water may possess a second critical point located at deeply supercooled conditions was formulated in an effort to provide a thermodynamically consistent interpretation for numerous experimentally-observed anomalies of water. While the preponderance of evidence is consistent with the existence of a second critical point, no unambiguous experimental proof has been found to date. Computer simulations can bypass the main challenge to experiments, rapid crystallization, but require computational efforts that prevented the rigorous verification of the presence of a second critical point in accurate water models up to now. Here, we use the histogram reweighting and large-system scattering calculations to investigate computationally two molecular models of water, TIP4P/2005 and TIP4P/Ice, widely regarded to be among the best classical force fields for this substance. We show that both models possess a metastable liquid-liquid critical point at deeply supercooled conditions and that this critical point is consistent with the 3-d Ising universality class. Next-generation challenges in this field include i) bringing higher accuracy (i.e. quantum mechanical accuracy) models to better performance so that above-mentioned analyses (finite-size scaling via histogram reweighting and large-system scattering) would be possible to perform ii) developing advanced sampling techniques to accelerate the sampling of slow-relaxation events, such as long-range correlations near criticality.
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