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.