Improving simulations of liquid-liquid separation in models of supercooled water

While water normally freezes at 0 degrees Celsius, it can be supercooled to a much lower temperature and still remain a liquid as long as no impurities exist on which ice crystals can form. A leading theory based on computational studies has suggested the possibility of a liquid-liquid transition in deeply supercooled water.

However, the majority of research has involved equation-of-state calculations or free energy studies, rather than the direct simulation of interfaces between phases. Direct simulations are typically restricted to a size on the order of 1,000 to 10,000 molecules due to limits on computational power. Working with such small systems makes it energetically costly for interfaces to form.

A new article reports a general thermodynamic compensation for this effect that allows for improved computational studies of interfaces in supercooled water and other systems. Singh et al. first derive the expression, an inequality for selecting the dimensions of the simulation cell, and then perform numerical tests of its validity for two water models. If the inequality is satisfied, the planar interface of the phase-separated fluid-fluid system remains stable with respect to the homogenous, single-phase state.

Previously, the authors’ free energy calculations pointed to a transition between low- and high-density liquid in deeply supercooled water. But in direct interface simulations, conflicting results on such phase separation were reported. The current work shows that sufficiently elongating the simulation cell is enough to stabilize the interface and uncover the expected result. The simulations by Singh et al. confirm the stability of a liquid-liquid interface in the ST2 model of water and show that if the thermodynamic criterion is followed, the same interface for the TIP4P/2005 model of water can be stabilized as well.

Source: “Thermodynamic analysis of the stability of planar interfaces between coexisting phases and its application to supercooled water,” by Rakesh S. Singh, Jeremy C. Palmer, Athanassios Z. Panagiotopoulos, and Pablo G. Debenedetti, The Journal of Chemical Physics (2019). The article can be accessed at http://doi.org/10.1063/1.5097591 .