>>13025669If the lowest-energy configuration of a bunch of particles happens to be a shape with a lot of empty space in the middle, then adding enough kinetic energy to keep the system from forming those high-volume shapes will tend to increase the density. For molecules that are mostly spherically symmetric, they will pack in one of the sphere packings, which are reasonably space-efficient, and so heating them up just adds jitter, which lowers density. Because water is polar, it’s lowest energy states have more structure to them than spherically symmetric molecules. For more detail, you’d have to actually work out the lowest-energy states
