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The paper identifies limitations in existing methods for quantifying rotational motion in molecular fluids, particularly in systems with complex dynamics like supercooled liquids. It argues that these methods fail to accurately capture the transition from diffusive fluid to arrested solid states, leading to inconsistencies in the literature. To address this, the authors introduce a new empirical method, benchmarked using continuous time random walk models, which accurately quantifies free and caged rotational motion, as well as non-Gaussian and non-Fickian dynamics.
Existing methods for quantifying molecular rotation break down when motion becomes complex, but this new method accurately captures rotational dynamics from fluid to solid states.
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.
