Structural anomalies of fluids: Origins in second and higher coordination shells

TL;DR

This study investigates how structural anomalies in fluids, related to changes in second and higher coordination shells, can be understood through molecular simulation and integral equation theory, aiding the prediction of anomalous fluid behavior.

Contribution

It demonstrates that integral equation theory effectively predicts structural anomalies in fluids based on pair potentials, confirming the microscopic role of coordination shells.

Findings

Structural anomalies linked to coordination shell changes are confirmed across multiple models.

Integral equation theory can predict anomalies from pair potentials efficiently.

Coordination shell contributions to excess entropy explain anomalous fluid properties.

Abstract

Compressing or cooling a fluid typically enhances its static interparticle correlations. However, there are notable exceptions. Isothermal compression can reduce the translational order of fluids that exhibit anomalous waterlike trends in their thermodynamic and transport properties, while isochoric cooling (or strengthening of attractive interactions) can have a similar effect on fluids of particles with short-range attractions. Recent simulation studies by Yan et al. [Phys. Rev. E 76, 051201 (2007)] on the former type of system and Krekelberg et al. [J. Chem. Phys. 127, 044502 (2007)] on the latter provide examples where such structural anomalies can be related to specific changes in second and more distant coordination shells of the radial distribution function. Here, we confirm the generality of this microscopic picture through analysis, via molecular simulation and integral equation theory, of coordination shell contributions to the two-body excess entropy for several related model fluids which incorporate different levels of molecular resolution. The results suggest that integral equation theory can be an effective and computationally inexpensive first-pass tool for assessing, based on the pair potential alone, whether new model systems are good candidates for exhibiting structural (and hence thermodynamic and transport) anomalies.