The EXP pair-potential system. I. Fluid phase isotherms, isochores, and quasiuniversality

TL;DR

This paper presents a detailed simulation study of the EXP pair-potential system, exploring its structure and dynamics in fluid phases, and demonstrates quasiuniversality through comparisons with Lennard-Jones systems and NVU-based proofs.

Contribution

It provides new insights into the structure and dynamics of the EXP system and offers a proof of quasiuniversality, extending understanding of simple liquids' behavior.

Findings

Radial distribution functions and mean-square displacement evolve predictably along isotherms and isochores.

The structure of Lennard-Jones systems can be approximated by EXP systems with the same reduced diffusion constant.

NVU-based proof supports the concept of quasiuniversality in the EXP system.

Abstract

The exponentially repulsive EXP pair potential defines a system of particles in terms of which simple liquids' quasiuniversality may be explained [A. K. Bacher et al., Nat. Commun. 5, 5424 (2014); J. C. Dyre, J. Phys. Condens. Matter 28, 323001 (2016)]. This paper and its companion present a detailed simulation study of the EXP system. Here we study how structure monitored via the radial distribution function and dynamics monitored via the mean-square displacement as a function of time evolve along the system's isotherms and isochores. The focus is on the gas and liquid phases, which are distinguished pragmatically by the absence or presence of a minimum in the radial distribution function above its first maximum. An NVU-based proof of quasiuniversality is presented, and quasiuniversality is illustrated by showing that the structure of the Lennard-Jones system at four selected state points is well approximated by those of EXP pair-potential systems with the same reduced diffusion constant. The companion paper studies the EXP system's isomorphs, focusing also on the gas and liquid phases.