Role of the first coordination shell in determining the equilibrium structure and dynamics of simple liquids

TL;DR

This study demonstrates that in simple liquids, including all interactions within the first coordination shell is crucial for accurate static and dynamic properties, challenging the traditional focus solely on repulsive forces.

Contribution

It shows that the first coordination shell determines the equilibrium structure and dynamics, emphasizing local interactions over the distinction between attractive and repulsive forces.

Findings

Interactions within the first coordination shell are essential for correct thermodynamics and dynamics.

Attractive forces can be ignored at high densities if all FCS interactions are included.

Results support the importance of local shells, aligning with van der Waals' theories.

Abstract

The traditional view that the physical properties of a simple liquid are determined primarily by its repulsive forces was recently challenged by Berthier and Tarjus, who showed that in some cases ignoring the attractions leads to large errors in the dynamics [L. Berthier and G. Tarjus, Phys. Rev. Lett. 103, 170601 (2009); J. Chem. Phys. 134, 214503 (2011)]. We present simulations of the standard Lennard-Jones liquid at several condensed-fluid state points, including a fairly low density state and a very high density state, as well as simulations of the Kob-Andersen binary Lennard-Jones mixture at several temperatures. By varying the range of the forces, results for the thermodynamics, dynamics, and structure show that the determining factor for getting the correct statics and dynamics is not whether or not the attractive forces {\it per se} are included in the simulations. What matters is whether or not interactions are included from all particles within the first coordination shell (FCS) - the attractive forces can thus be ignored, but only at extremely high densities. The recognition of the importance of a local shell in condensed fluids goes back to van der Waals; our results confirm this idea and thereby the basic picture of the old hole- and cell theories for simple condensed fluids.