Simple Physical Explanation of the Unusual Thermodynamic Behavior of Liquid Water

Here we demonstrate that the unusual thermodynamic behavior of water can be explained by a simple two-order-parameter Landau-type theory naturally and physically, without considering the effects of a liquid-liquid critical point. To describe the hydrogen-bonding effects on the phase behavior, we introduce a bond order parameter $S$, in addition to the density order parameter $\rho$. We ascribe the unusualness of water to the competing orderings of $\rho$ and $S$, or their strong negative coupling. We argue that in usual liquids the crystallization is primarily a result of the ordering of $\rho$, while in water it is a result of the ordering of $S$ at ambient pressure.