Supercooled Water Simulations Reveal the Hidden Physics of Glassy Arrest

Supercooled water undergoes a kinetic freeze near 220 Kelvin, where molecular motion suddenly shifts from equilibrium dynamics to glassy arrest. This temperature region overlaps directly with the structural transformation between high-density liquid and low-density liquid configurations, making it notoriously difficult to study experimentally.

To untangle this complex thermodynamic behavior, researcher Vitaly Kocherbitov utilized extensive molecular dynamics simulations to model the thermal properties of water. The work, published in Scientific Reports, demonstrates how equilibrium thermodynamics and nonequilibrium glass dynamics interact during rapid cooling and heating scans.

How Varying Scan Speeds Reveal Equilibrium

Water exhibits numerous anomalies, such as an exceptionally high heat capacity and a density maximum at 277 Kelvin under ambient pressure. When scientists cool water below its normal freezing point without crystallization, these unusual properties become even more pronounced. However, experimental work in this regime is heavily restricted by spontaneous ice formation in what researchers call the no man’s land.

To bypass crystallization and probe microscopic mechanisms, computational models are essential. The author simulated a large box containing 4,096 water molecules using the TIP4P/2005 water model, applying cooling and heating scan rates spanning from 10 to 3,000 Kelvin per microsecond. At temperatures above approximately 220 Kelvin, both molar volume and enthalpy remained completely independent of the cooling rate. This independence confirms that the simulations capture true equilibrium thermodynamic behavior in that temperature range.

Below 220 Kelvin, however, the curves begin to diverge based on how quickly the system is cooled. Faster scans deviate first, reflecting the onset of kinetic arrest where molecules can no longer rearrange fast enough to maintain thermal equilibrium.

Two Distinct Viscosity Regimes Shape Liquid Dynamics

The sluggish dynamics of supercooled water are reflected in a sharp increase in viscosity and a concurrent drop in the self-diffusion coefficient. By calculating viscosity through perturbation and fluctuation methods, the simulations uncovered two distinct activation energy regimes operating in tandem.

At higher temperatures in the high-density liquid state, viscosity follows a low-temperature process with an activation energy of roughly 20.7 kilojoules per mole, which aligns closely with the energy of a single hydrogen bond. This regime is governed by the structural switching mechanism where hydrogen bond donors transition from overcoordinated to undercoordinated oxygen atoms.

At lower temperatures, a second process takes over with an activation energy of 85.6 kilojoules per mole, about four times higher than the first. The viscosity across the entire liquid range can be accurately modeled as the sum of these two parallel contributions. As water cools, structural analysis shows a steady depletion of both overcoordinated and undercoordinated oxygen atoms, effectively shutting down the high-temperature relaxation pathway and forcing the system into cooperative, high-activation-energy rearrangements.

Separating Thermodynamics From Glassy Arrest

A major challenge in water science has been reconciling simulated glass transition temperatures with laboratory measurements. Laboratory differential scanning calorimetry typically uses slow scan rates around 10 Kelvin per minute, which shifts observed glass transition temperatures down by dozens of degrees compared to ultrafast computer simulations.

To bridge this gap, the author implemented a combined structural evolution and glass transition model. This framework successfully separates the thermodynamic heat-capacity peak associated with the high-to-low density liquid transition from the dynamic heat-capacity overshoot caused by enthalpy recovery during glass softening.

When extrapolated to standard laboratory time scales of 10 Kelvin per minute, the model predicts a glass transition onset temperature of 136 Kelvin. This matches the widely accepted experimental value historically measured for amorphous solid water. The findings confirm that the prominent heat capacity anomalies seen in fast simulations stem from an intricate interplay between equilibrium structural evolution and timescale-dependent kinetic freezing, rather than a distinct phase change.

The research was published in Scientific Reports on November 6, 2026.