Statistical entropy and density maximum anomaly in liquid water

We studied the configurational entropy of liquid water at ambient pressure and extracted the cumulative contribution associated with spatial correlations involving more than two particles. This quantity, the so-called residual multi-particle entropy (RMPE), is a sensitive indicator of structural modifications and incoming thermodynamic transitions, being positive in the more disordered phase and negative in the more ordered one. The vanishing of the RMPE has been successfully exploited as a rather general ordering criterion in a variety of model fluid systems. We carried out molecular-dynamics calculations on the TIP4P model at ambient pressure for temperatures between 230 K and 350 K and found that the ordering threshold estimated through the zero-RMPE criterion falls close to the temperature of maximum density.