AI has finally decoded the 100-year-old mystery of why water is densest at 4° Celsius.
March 31, 2026
Original Paper
Understanding the Density Maximum of Water with Machine Learned Potentials
arXiv · 2603.27767
The Takeaway
While most liquids get denser as they get colder, water hits its maximum density just before it turns to ice. Researchers used machine learning to find that this happens because the molecular structure 'collapses' at a specific distance while staying perfectly ordered nearby.
From the abstract
After melting, at ambient pressure, the density of water continues to increase with temperature until it reaches a maximum around 4 °C. For nearly a century, this phenomenon has been qualitatively attributed to a mixture of ordered and disordered structures. Herein, we employ a deep neural network to train a machine learned (ML) interatomic potential for water using electronic structure data from advanced density functional theory. Notably, molecular dynamics simulations with the ML potential re