Computational probes of molecular motion in the Lewis and Whanstrom model for ortho-terphenyl

TL;DR

This study uses molecular dynamics simulations to analyze translational and rotational diffusion in a model of ortho-terphenyl, revealing non-Gaussian behavior, spatial heterogeneity, and a supercooling-induced decoupling of translation and rotation.

Contribution

It introduces an Einstein-based approach for rotational motion analysis and demonstrates its advantages over the Debye model at supercooled conditions.

Findings

Rotational motion deviates from Debye model at low temperatures.

Spatially heterogeneous dynamics are observed in both translation and rotation.

Supercooling enhances the effective rotational motion relative to translation.

Abstract

We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260 K < T < 346 K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly-used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, which strengthens upon supercooling. Examination of particle mobility reveals spatially heterogeneous dynamics in translation and rotation, with a strong spatial correlation between translationally and rotationally mobile particles. Application of the Einstein formalism to the analysis of translation-rotation decoupling results in a trend opposite to that seen in conventional approaches based on the Debye formalism, namely an enhancement in the effective rate of rotational motion relative to translation upon supercooling.