Anomalous structure and dynamics of the Gaussian-core fluid

TL;DR

This study explores the anomalous structural and dynamic behaviors of the Gaussian-core fluid, revealing how temperature and density influence its properties and uncovering relationships with excess entropy and differences from waterlike fluids.

Contribution

It demonstrates the existence of a structural order anomaly in the Gaussian-core fluid and links it to excess entropy, highlighting differences from waterlike fluids and the role of higher-body correlations.

Findings

Structural order decreases upon compression under certain conditions.

Self-diffusivity scales with two-body excess entropy similarly to simple liquids.

Anomalies form cascading regions on the temperature-density plane.

Abstract

It is known that there are thermodynamic states for which the Gaussian-core (GC) fluid displays anomalous properties such as expansion upon isobaric cooling (density anomaly) and increased single-particle mobility upon isothermal compression (self-diffusivity anomaly). We investigate how temperature and density affect its short-range translational structural order, as characterized by the two-body excess entropy. We find that there is a wide range of conditions for which the short-range translational order of the GC fluid decreases upon isothermal compression (structural order anomaly). The origin of the structural anomaly is qualitatively similar to that of other anomalous fluids and is connected to how compression affects static correlations at different length scales. We find that the self-diffusivity of the GC fluid obeys a scaling relationship with the two-body excess entropy that is very similar to the one observed for a variety of simple liquids. One consequence of this relationship is that the state points for which structural, self-diffusivity, and density anomalies of the Gaussian-core fluid occur appear as cascading regions on the temperature-density plane, a phenomenon observed earlier for models of waterlike fluids. There are, however, key differences between the anomalies of GC and waterlike fluids, and we discuss how those can be qualitatively understood by considering the respective interparticle potentials of these models. Finally, we note that the self-diffusivity of the Gaussian-core fluid obeys different scaling laws depending on whether the two-body or total excess entropy is considered. This finding, which deserves more comprehensive future study, appears to underscore the significance of higher-body correlations for the behavior of fluids with bounded interactions.