$NVU$ view on energy polydisperse Lennard-Jones systems

TL;DR

This paper demonstrates that energy polydispersity in Lennard-Jones systems has minimal impact on structure and dynamics due to invariance of the potential-energy surface, unlike size polydispersity.

Contribution

The study introduces the NVU approach to explain the invariance of energy polydisperse Lennard-Jones systems and validates it through simulations up to 30% polydispersity.

Findings

Energy polydispersity causes little change in structure and dynamics.

Invariance of the potential-energy surface underpins the similar physics to single-component systems.

Size polydispersity significantly alters the potential-energy surface.

Abstract

When energy polydispersity is introduced into the Lennard-Jones (LJ) system, there is little effect on structure and dynamics [Ingebrigtsen and Dyre, J. Phys. Chem. B 127, 2837 (2023)]. For instance, at a given state point both the radial distribution function and the mean-square displacement as a function of time are virtually unaffected by even large energy polydispersity, which is in stark contrast to what happens when size polydispersity is introduced. We here argue -- and validate by simulations of up to 30\% polydispersity -- that this almost invariance of structure and dynamics reflects an approximate invariance of the constant-potential-energy surface. Because $NVU$ dynamics defined as geodesic motion at constant potential energy is equivalent to Newtonian dynamics in the thermodynamic limit, the approximate invariance of the constant-potential-energy surface implies virtually the same physics of energy polydisperse LJ systems as of the standard single-component version. In contrast, the constant-potential-energy surface is significantly affected by introducing size polydispersity.