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Enhancement of the thermal polarization of water via heat flux and dipole moment dynamic correlations.

It has been recently shown that liquid water polarizes as a response to a temperature gradient. This polarization effect can be significant for temperature gradients that can be achieved at micro and nanoscales. In this paper we investigate the dependence of the polarization response of liquid and supercritical water at different thermodynamic conditions using both equilibrium and nonequilibrium molecular dynamics simulations for the extended point charge water model. We find that the thermal polarization features a nonmonotonic behavior with temperature, reaching a maximum response at specific thermodynamic states. We show that the thermal polarization is maximized when the density of states of the heat flux and dipole moment correlation functions feature the strongest overlap. The librational modes of water are shown to play an important role in determining this behavior as well as the heat transport mechanism in water. The librational frequencies show a significant dependence with temperature and pressure. This dependence provides a microscopic mechanism to explain the observed maximization of the thermal-polarization effect. Our work provides new microscopic insights on the mechanism determining the orientation of polar fluids under thermal gradients, as well as new strategies to maximize their orientation by manipulating the dynamic correlations between the heat flux and the sample dipole moment.