Isomorphs in the phase diagram of a model liquid without inverse power law repulsion

TL;DR

This study shows that liquids with Buckingham potential exhibit strong virial-potential energy correlations and possess isomorphs, indicating hidden scale invariance and similar dynamics to inverse power-law systems, despite different repulsion forms.

Contribution

It demonstrates that Buckingham liquids have isomorphs and strong correlations, extending the concept beyond inverse power-law interactions to include exponential repulsions.

Findings

Buckingham liquids are strongly correlating.

They possess isomorphs with invariant structure and dynamics.

Their behavior can be approximated by inverse power-law models.

Abstract

It is demonstrated by molecular dynamics simulations that liquids interacting via the Buckingham potential are strongly correlating, i.e., have regions of their phase diagram where constant-volume equilibrium fluctuations in the virial and potential energy are strongly correlated. A binary Buckingham liquid is cooled to a viscous phase and shown to have isomorphs, which are curves in the phase diagram along which structure and dynamics in appropriate units are invariant to a good approximation. To test this, the radial distribution function, and both the incoherent and coherent intermediate scattering function are calculated. The results are shown to reflect a hidden scale invariance; despite its exponential repulsion the Buckingham potential is well approximated by an inverse power-law plus a linear term in the region of the first peak of the radial distribution function. As a consequence the dynamics of the viscous Buckingham liquid is mimicked by a corresponding model with purely repulsive inverse-power-law interactions. The results presented here closely resemble earlier results for Lennard-Jones type liquids, demonstrating that the existence of strong correlations and isomorphs does not depend critically on the mathematical form of the repulsion being an inverse power law.