Structure–dynamics correlation and its link to fragility and dynamic heterogeneity

Understanding the connection between structure, dynamics, and fragility, the rate at which the relaxation time grows with the decreasing temperature, is central to unravelling the glass transition. Fragility is often associated with dynamic heterogeneity, implying that if the structure influences the dynamics, more fragile systems should exhibit stronger structure–dynamics correlations. In this study, we test the generality of this assumption using the Lennard–Jones (LJ) and Weeks–Chandler–Andersen systems, where the fragility is tuned via the density, and a modified LJ ( q , p ) system, where the fragility is varied by changing the potential softness. We define a structural order parameter based on a mean-field caging potential and analyse the energy barriers at both the macroscopic and microscopic levels. While the macroscopic free energy barrier slope correlates with fragility, the microscopic free energy barrier does not show a consistent trend. Instead, it exhibits a strong correlation with a structure–dynamics correlation measure obtained from isoconfigurational ensemble simulations. Interestingly, the two systems showing the highest structure–dynamics correlation, LJ at ρ = 1.1 and the (8, 5) model, are respectively the least and most fragile within their classes. These systems exhibit broad mobility distributions and large non-Gaussian parameters but low four-point susceptibilities, suggesting a decoupling between the spatial correlation length and mobility contrast. Both systems lie in the enthalpy-dominated regime and are close to the spinodal, pointing to mechanical instability as a source of heterogeneity. Our results reveal that the structure–dynamics correlation is more closely linked to the contrast in individual particle mobility than to the spatial extent of dynamic correlations that typically scale with fragility.