"The Microscopic Physical Cause for the Density Maximum of Liquid Water"Philipp Tröster and Paul Tavan
J. Phys. Chem. Lett. 5, 138−142 (2014).
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.
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Polarizable force fields for molecular mechanics from first principles