Molecular motion at the experimental glass transition

TL;DR

This paper introduces a new Monte Carlo simulation method that significantly speeds up the study of molecular glass transition, enabling analysis of equilibrium structures and dynamics at experimental conditions.

Contribution

The authors develop a flip Monte Carlo algorithm for molecular models that accelerates sampling by a factor of about 10^9 at the glass transition temperature, allowing detailed analysis of molecular glass behavior.

Findings

The method achieves a sampling speedup of about 10^9 at T_g.

Results show closer alignment with experimental fragility and Stokes-Einstein relation deviations.

Spatial correlations and molecular motions near T_g are visualized and characterized.

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 ortho-terphenyl 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 $10^9$ at the experimental glass transition temperature $T_g$. 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 $T_g$. Our approach can be generalized to a \rev{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.