Explaining why simple liquids are quasi-universal

TL;DR

This paper investigates the quasi-universality of simple liquids, demonstrating that liquids with pair potentials approximable by sums of exponential terms exhibit similar structure and dynamics, and conjecturing this as a defining feature.

Contribution

It establishes a theoretical criterion linking quasi-universality to the form of pair potentials, specifically sums of exponential functions with large prefactors.

Findings

Liquids with pair potentials as sums of exponentials show similar structure and dynamics.

The exponentially repulsive potential exhibits strong virial potential-energy correlations.

Conjecture that quasi-universality only occurs for systems with such pair potentials.

Abstract

It has been known for a long time that many simple liquids have surprisingly similar structure as quantified, e.g., by the radial distribution function. A much more recent realization is that the dynamics are also very similar for a number of systems with quite different pair potentials. Systems with such non-trivial similarities are generally referred to as "quasi-universal". From the fact that the exponentially repulsive pair potential has strong virial potential-energy correlations in the low-temperature part of its thermodynamic phase diagram, we here show that a liquid is quasi-universal if its pair potential can be written approximately as a sum of exponential terms with numerically large prefactors. Based on evidence from the literature we moreover conjecture the converse, i.e., that quasi-universality only applies for systems with this property.