Excitation Populations Provide a Thermodynamic Order Parameter for Liquids

Recent simulation and model system studies suggest that local structural excitations play an important role in the dynamics of liquids and glasses. Here, for the first time, we quantify excitation populations in real liquids, showing that their temperature-dependent population can be predicted from entropy and enthalpy of melting. We further show that the excitation population in the first solvent shell serves as an order parameter for the appearance of dynamic heterogeneity and for driving transformations between distinct mechanistic regimes of liquid relaxation. We propose a scenario that provides simple physical explanations for these previously enigmatic aspects of liquid behavior.