Models for a liquid-liquid phase transition

TLDR: In this article, the authors use molecular dynamics simulations to study two-and three-dimensional models with the isotropic double-step poten tial which in addition to the hard core has a repulsive soft core of larger radius.

Abstract: We use molecular dynamics simulations to study two- and three-dimensional models with the isotropic double-step poten tial which inadditionto the hard core has a repulsive soft core of larger radius. Our results indicate that the presence of two characteristic repulsive distances (hard core and soft core) is su8cient to explain liquid anomalies and a liquid–liquid phase transition, but these two phenomena may occur independently. Thus liquid–liquid transitions may exist in systems like liquid metals, regardless of the presence of the density anomaly. For 2D, we propose a model with a speci;c set of hard core and soft core parameters, that qualitatively reproduces the phase diagram and anomalies of liquid water. We identify two solid phases: a square crystal (high density phase), and a triangular crystal (low density phase) and discuss the relationbetweenthe an omalies of liquid an d the polymorphism of the solid. Similarly to real water, our 2D system may have the second critical point in the metastable liquid phase beyond the freezing line. In 3D, we ;nd several sets of parameters for which two >uid–>uid phase transition lines exist: the ;rst line between gas and liquid and the second line between high-density liquid (HDL) and low-density liquid (LDL). In all cases, the LDL phase shows no density anomaly in 3D. We relate the absence of the density anomaly with the positive slope of the LDL–HDL phase transition line. c