The Microscopic Physical Cause for the Density Maximum of Liquid Water

The existence of a density maximum at 277 K is probably the most prominent anomaly among the many very special thermodynamic properties of liquid water. While usually attributed to so-called hydrogen bonding, the microscopic physical cause of this prominent anomaly is still elusive. Here we show that the density anomaly is caused by those short-range electrostatic forces, which are generated by the quadrupole and higher moments of the charge distributions present in liquid-phase water molecules. This conclusion derives from 20 ns replica exchange molecular-dynamics simulations with closely related polarizable four-, five-, and six-point water models. As soon as the model complexity suffices to represent the higher electrostatic moments with sufficient accuracy, the density temperature profile n(T) calculated for T ∈ [250,320] K at the standard pressure 1 bar locks in to the experimental observation. The corresponding six-point model is, therefore, the most simple available cartoon for liquid-phase water molecules.