Relationship between the potential energy landscape and the dynamic crossover in a water-like monatomic liquid with a liquid-liquid phase transition.

In the case of fragile liquids, dynamical properties such as the structural relaxation time evolve from Arrhenius at high-temperatures to non-Arrhenius at low temperatures. Computational studies show that (i) in the Arrhenius dynamic domain, the liquid samples regions of the potential energy landscape (PEL) that are insensitive to temperature (PEL-independent regime) and the relaxation is exponential, while (ii) in the non-Arrhenius dynamic domain, the topography of the PEL explored by the liquid varies with temperature (PEL-influenced regime) and the relaxation is non-exponential. In this work we explore whether the correlation between dynamics and PEL regimes, points (i) and (ii), holds for the Fermi-Jagla (FJ) liquid. This is a monatomic model liquid that exhibits many of the water anomalous properties, including maxima in density and diffusivity. The FJ model is a rather complex liquid that exhibits a liquid-liquid phase transition and a liquid-liquid critical point (LLCP), as hypothesized for the case of water. We find that, for the FJ liquid, the correlation between dynamics and the PEL regimes is not always present and depends on the density of the liquid. For example, at high density, the liquid exhibits Arrhenius/non-Arrhenius (AnA) dynamical crossover, exponential/non-exponential (EnE) relaxation crossover, and a PEL-independent/PEL-influenced regime crossover, consistent with points (i) and (ii). However, in the vicinity of the LLCP, the AnA crossover is absent but the liquid exhibits EnE relaxation and PEL regime crossovers. At very low density, crystallization intervenes and the PEL regime crossover is suppressed. Yet, the AnA dynamical crossover and the EnE relaxation crossover remain. It follows that the dynamics in liquids (AnA and EnE crossovers) are not necessarily correlated with the changes between the PEL regimes, as one could have expected. Interestingly, the AnA crossover in the FJ liquid is not related to the presence of the Widom line. This result may seem to be at odds with previous studies of polymorphic model liquids, and a simple explanation is provided.