How to quantify structural anomalies in fluids?

TL;DR

This paper compares various methods for quantifying structural anomalies in fluids and finds that excess entropy provides the most reliable measure, challenging some traditional definitions.

Contribution

It provides the first explicit comparison of different structural order measures and advocates for excess entropy as the most consistent indicator of anomalies.

Findings

Some definitions contradict intuitive fluid behavior

Excess entropy correlates best with structural anomalies

Traditional measures may be unreliable for quantification

Abstract

Some fluids are known to behave anomalously. The so-called structural anomaly which means that the fluid becomes less structures under isothermal compression is among the most frequently discussed ones. Several methods for quantifying the degree of structural order are described in the literature and are used for calculating the regions of structural anomalies. It is implied that all of the structural order determinations yield qualitatively identical results. However, no explicit comparison was made. This paper presents such a comparison for the first time. the results of some definitions are shown to contradict the intuitive notion of a fluid. On the basis of this comparison we show that the structural anomaly can be most reliably determined from the behavior of the excess entropy.