A comprehensive scenario of the thermodynamic anomalies of water using the TIP4P/2005 model

Abstract

The striking behavior of water has deserved it to be referred to as an "anomalous" liquid. The water anomalies are greatly amplified in metastable (supercooled/stretched) regions. This makes difficult a complete experimental description since, beyond certain limits, the metastable phase necessarily transforms into the stable one. Theoretical interpretation of the water anomalies could then be based on simulation results of well validated water models. But the analysis of the simulations has not yet reached a consensus. In particular, one of the most popular theoretical scenarios -involving the existence of a liquid-liquid critical point (LLCP)- is disputed by several authors. In this work we propose to use a number of exact thermodynamic relations which can be tested in a region of the phase diagram outside the LLCP thus avoiding the problems associated to the coexistence region. The central property connected to other water anomalies is the locus of temperatures at which the density along isobars attain a maximum (TMD line) or a minimum (TmD), computed via simulations for a successful water model, TIP4P/2005. Next, we have evaluated the vapor-liquid spinodal in the region of large negative pressures. The shape of these curves and their connection to the extrema of response functions (isothermal compressibility and heat capacity at constant pressure) may help to elucidate the validity of the theoretical proposals. In this way we are able to present for the first time a comprehensive scenario of the thermodynamic water anomalies for TIP4P/2005 and their relation to the vapor-liquid spinodal. The overall picture shows a remarkable similarity with the corresponding one for the ST2 water model, for which the existence of a LLCP has been demonstrated in recent years. It also provides a hint as to where the long-sought for extrema in response functions might become accessible to experiments.