Simulation and theoretical analysis of the origin of the temperature of maximum density of water

The presence of a maximum in density is one of the well-known anomalies of water. This maximum is located at 4 degrees C at room pressure and it moves to lower temperatures as pressure increases. In this work, we attempt to provide a quantitative explanation of the existence of the temperature of maximum density, T-MD, and its dependence on pressure, using a description in terms of low and high density water states. To that aim, we analyzed NpT molecular dynamics trajectories of TIP4P/2005 liquid water from -1800 bar to 1000 bar, and from 238.15 K to 328.15 K. We computed the mole fractions of the low and high density water states and their partial molar volumes. We found that the temperature dependence of both quantities plays a fundamental role in the existence of the T-MD. On the other hand, only the variation with pressure of the mole fractions of the low and high density water is essential to explain the T-MD pressure dependence. Additionally, we have also performed an equivalent analysis using the parameterization of the two-state thermodynamic model proposed by Biddle et al. [J. Chem. Phys. 146, 034502 (2017)] obtaining similar conclusions.

Automated summary

This study provides a quantitative explanation for the existence and dependence of the temperature of maximum density (T-MD) on pressure in water. The analysis reveals that the temperature dependence of low and high density water states, as well as the variation of mole fractions with pressure, play crucial roles in the T-MD phenomenon.