How to quantify long-time rotational motion in molecular systems

TL;DR

This paper identifies limitations in existing methods for quantifying complex rotational motion in molecular systems and introduces a new empirical approach that accurately captures diverse rotational dynamics, especially in supercooled liquids.

Contribution

The authors develop and benchmark a novel empirical method that overcomes previous limitations, enabling accurate analysis of complex rotational dynamics in molecular fluids.

Findings

Existing methods fail in complex rotational systems near glass transition.

The new method accurately captures non-Gaussian and non-Fickian rotational behaviors.

Benchmarking shows the method effectively characterizes dynamic heterogeneity.

Abstract

We show that all existing methods quantifying rotational motion in molecular fluids eventually fail in systems undergoing complex rotational motion characterised by slow, heterogeneous, or intermittent dynamics. This impacts in particular the study of rotational dynamics in molecular supercooled liquids near their glass transition, as well as discussions of the decoupling between rotational and translational motion and violations of the Debye-Stokes-Einstein relation. We present a brief overview of existing methods and explain why none of them can accurately capture the evolution of rotational dynamics from a diffusive fluid to an arrested solid, thus resolving inconsistent literature results. We then introduce an empirical method that efficiently solves all issues. We benchmark our method devising a family of continuous time random walk models for rotational dynamics. Our method correctly quantifies the statistics of free and caged rotational motion, as well as non-Gaussian and non-Fickian rotational dynamics, and should allow a better characterisation of dynamic heterogeneity in the rotational motion of supercooled molecular fluids.