Pressure-energy correlations in liquids. I. Results from computer simulations

TL;DR

This study demonstrates that many simple liquids exhibit strong pressure-energy correlations in simulations, linked to effective inverse power-law potentials, while complex liquids like water do not show such correlations.

Contribution

It provides detailed simulation results showing pressure-energy correlations in various model liquids and identifies the conditions under which these correlations are significant.

Findings

Strong correlations in Lennard-Jones liquids and some simple models.

No significant correlations in hydrogen-bonding liquids like water.

Correlations linked to effective inverse power-law potentials.

Abstract

We show that a number of model liquids at fixed volume exhibit strong correlations between equilibrium fluctuations of the configurational parts of (instantaneous) pressure and energy. We present detailed results for thirteen systems, showing in which systems these correlations are significant. These include Lennard-Jones liquids (both single- and two-component) and several other simple liquids, but not hydrogen-bonding liquids like methanol and water, nor the Dzugutov liquid which has significant contributions to pressure at the second nearest neighbor distance. The pressure-energy correlations, which for the Lennard-Jones case are shown to also be present in the crystal and glass phases, reflect an effective inverse power-law potential dominating fluctuations, even at zero and slightly negative pressure. An exception to the inverse-power law explanation is a liquid with hard-sphere repulsion and a square-well attractive part, where a strong correlation is observed, but only after time-averaging. The companion paper [arXiv:0807.0551] gives a thorough analysis of the correlations, with a focus on the Lennard-Jones liquid, and a discussion of some experimental and theoretical consequences.