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Molecular Motion at the Experimental Glass Transition

Romain Simon1, Jean-Louis Barrat2, and Ludovic Berthier3

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Phys. Rev. X 16, 011035 – Published 24 February, 2026

DOI: https://doi.org/10.1103/4twk-33j7

Abstract

We propose a novel computational strategy to study the glass transition of molecular fluids. Our approach combines the construction of simple yet realistic models with the development of Monte Carlo algorithms to accelerate equilibration and sampling. Inspired by the well-studied orthoterphenyl glass former, we construct a molecular model with an analogous triangular geometry and construct a “flip” Monte Carlo algorithm. We demonstrate that the flip Monte Carlo algorithm achieves a sampling speedup of about 109 at the experimental glass transition temperature Tg. This allows us to systematically analyze the equilibrium structure and molecular dynamics of the model over a temperature regime previously inaccessible. We carefully compare the observed physical behavior to earlier studies that used atomistic models. In particular, we find that the glass fragility and the departure from the Stokes-Einstein relation are much closer to experimental observations. We characterize the development and temperature evolution of spatial correlations in the relaxation dynamics using both orientational and translational degrees of freedom. Excess wings emerge at intermediate frequencies in dynamic rotational spectra, and we directly visualize the corresponding molecular motion near Tg. Our approach can be generalized to a broad range of molecular geometries and paves the way to a deeper understanding of how molecular details may affect more universal physical aspects characterizing molecular liquids approaching their glass transition.

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The glass transmission remains a central challenge because molecular liquids slow down by many orders of magnitude without obvious structural changes. Computational methods based on swap Monte Carlo have transformed our understanding of glassy liquids, but they were limited to simple pointlike particles. We introduce a general computational strategy that extends this approach to molecular systems and enables simulations directly at the experimental glass transition. Using a simple yet realistic molecular model together with a tailored Monte Carlo algorithm, we accelerate equilibration by many orders of magnitude while preserving realistic molecular motion. This allows direct access to equilibrium relaxation, spatial correlations, and transport anomalies at the experimental glass transition temperature. Although we focus on a single molecular geometry, the strategy is broadly applicable and can be adapted to many molecular shapes and interactions. Opening this regime to simulation provides a versatile framework for studying both universal and molecule specific aspects of glassy dynamics.

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Phys. Rev. X 16, 011035– Published 24 February, 2026
  • Received 4 July 2025
  • Accepted 23 December 2025
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