A pot of water on the stove is just water, but inside each molecule, a continuous dance goes on. Hydrogen and oxygen atoms jitter, even at absolute zero, because that's how the quantum world works. These jitters change how much heat water stores and how it flows. Previously, accounting for them required incredibly heavy calculations: each atom turned into a whole fan of its copies, and the computer would choke.

The authors of the new work found a loophole. They take an expensive quantum calculation but use it only as a teacher. The teacher shows how water behaves with jitters, and the student, a simple classical model, learns these lessons. Then the student works on its own, fast and almost cost-free, but with an error smaller than the thickness of a hair compared to a room. In the paper, they tested the method on water and other systems where molecules are held together by hydrogen bonds. Result: the energy matches the exact quantum calculation to within 0.2 millielectronvolts per atom. That's like measuring the height of Everest to the accuracy of a grain of sand.

The method is called EPIGS (entropic path-integral coarse-graining). The main trick is that it can transfer what it learned to other temperatures and system sizes. Train on a tiny cube of water — apply to a whole lake. This opens the door to studying proteins, drugs, and materials where quantum effects matter but were previously inaccessible. The pot on the stove has become a bit more understandable, and so has everything that contains water.