Scaling of the dynamics of flexible Lennard-Jones chains. II. Effects of harmonic bonds

TL;DR

This study explores how harmonic bonds affect the scaling and invariance properties of Lennard-Jones chain liquids, revealing the existence of pseudoisomorphs where structure and dynamics are invariant but excess entropy is not.

Contribution

It demonstrates that harmonic bonds disrupt potential energy-virial correlations, leading to pseudoisomorphs where only some properties remain invariant, extending the isomorph theory.

Findings

Harmonic bonds weaken energy-virial correlations.

Pseudoisomorphs exist with invariant structure and dynamics.

Excess entropy is not invariant on these curves.

Abstract

The previous paper [Veldhorst et al., J. Chem. Phys. 141, 054904 (2014)] demonstrated that the isomorph theory explains the scaling properties of a liquid of flexible chains consisting of ten Lennard-Jones particles connected by rigid bonds. We here investigate the same model with harmonic bonds. The introduction of harmonic bonds almost completely destroys the correlations in the equilibrium fluctuations of the potential energy and the virial. According to the isomorph theory, if these correlations are strong a system has isomorphs, curves in the phase diagram along which structure, dynamics and the excess entropy are invariant. The Lennard-Jones chain liquid with harmonic bonds does have curves in the phase diagram along which the structure and dynamics are invariant. The excess entropy is not invariant on these curves, which we refer to as "pseudoisomorphs". In particular this means that Rosenfeld's excess-entropy scaling (the dynamics being a function of excess entropy only) does not apply for the Lennard-Jones chain with harmonic bonds.