The quantum mechanics behind water’s unique thermal properties

Water’s exceptionally high heat capacity is one of its many remarkable properties, helping it regulate temperatures on scales ranging from biological processes to global climate. Though these properties can be experimentally measured, their origins are not known, and they are computationally expensive to simulate. By replacing ab initio energies with machine learning potentials in the simulations, Shiga et al. reduced the computational cost by orders of magnitude, unlocking new levels of detail about water’s thermodynamic properties that were previously inaccessible.

“Knowing the macroscopic properties alone — like heat capacity — does not reveal the microscopic mechanisms that give rise to them,” said author Motoyuki Shiga. “Understanding the underlying origin, such as how hydrogen bonding and quantum fluctuations contribute, allows us to predict how water behaves under different conditions, design better models, and extend insights to aqueous solutions, biological systems, or climate models.”

Accurately simulating water’s microscopic mechanisms requires reproducing the interplay between its quantum effects and structural flexibility. The flexibility of its hydrogen bonds allows them to absorb energy instead of breaking, causing them to act as a thermal reservoir. On the other hand, quantum effects, which are neglected by most simulations, suppress the system’s energy absorption efficiency, causing an overestimation of heat capacity.

To address this, the researchers trained their system with six different combinations of water temperature and pressure and used accurate descriptions of water’s electronic structure and interatomic forces. They also explicitly incorporated nuclear quantum effects.

“Together, they allow a fully quantum mechanical treatment,” said Shiga.

They plan to extend their work to include water under constant pressure to investigate how its properties depend on temperature. They also plan to study aqueous solutions and interfaces, which are important for biological environments.

Source: “Computation of the heat capacity of water from first principles,” by Motoyuki Shiga, Jan Elsner, Jörg Behler, and Bo Thomsen, Journal of Chemical Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0285698 .

This paper is part of the Michele Parrinello Festschrift Collection, learn more here .