Mode coupling theory for molecular liquids: What can we learn from a system of hard ellipsoids?

TL;DR

This paper compares molecular dynamics and Percus-Yevick theory for hard ellipsoids and Lennard-Jones molecules to understand the coupling of translational and orientational degrees of freedom near the glass transition.

Contribution

It demonstrates that hard ellipsoids can serve as a simplified model to study complex molecular glass transition behaviors.

Findings

Static correlators are similar at the glass transition for both systems.

Non-ergodicity parameters show remarkable similarity when aspect ratio is chosen properly.

Hard ellipsoids effectively model key behaviors of Lennard-Jones molecules near the glass transition.

Abstract

Molecular fluids show rich and complicated dynamics close to the glass transition. Some of these observations are related to the fact that translational and orientational degrees of freedom couple in nontrivial ways. A model system which can serve as a paradigm to understand these couplings is a system of hard ellipsoids of revolution. To test this we compare at the ideal glass transition the static molecular correlators of a linear A-B Lennard-Jones molecule obtained from a molecular dynamics simulation with a selected fluid of hard ellipsoids for which the static correlators have been obtained using Percus-Yevick theory. We also demonstrate that the critical non-ergodicity parameters obtained from molecular mode coupling theory for both systems show a remarkable similarity at the glass transition, provided the aspect ratio is chosen properly. Therefore we conclude that a system of hard ellipsoids can indeed be used to understand part of the essential behaviour of such a simple molecular system like the A-B Lennard-Jones molecules in the vicinity of the ideal glass transition.