Evidence of a liquid-liquid phase transition in H[Formula: see text]O and D[Formula: see text]O from path-integral molecular dynamics simulations.

ABSTRACT

We perform path-integral molecular dynamics (PIMD), ring-polymer MD (RPMD), and classical MD simulations of H[Formula see text]O and D[Formula see text]O using the q-TIP4P/F water model over a wide range of temperatures and pressures. The density [Formula see text], isothermal compressibility [Formula see text], and self-diffusion coefficients D(T) of H[Formula see text]O and D[Formula see text]O are in excellent agreement with available experimental data; the isobaric heat capacity [Formula see text] obtained from PIMD and MD simulations agree qualitatively well with the experiments. Some of these thermodynamic properties exhibit anomalous maxima upon isobaric cooling, consistent with recent experiments and with the possibility that H[Formula see text]O and D[Formula see text]O exhibit a liquid-liquid critical point (LLCP) at low temperatures and positive pressures. The data from PIMD/MD for H[Formula see text]O and D[Formula see text]O can be fitted remarkably well using the Two-State-Equation-of-State (TSEOS). Using the TSEOS, we estimate that the LLCP for q-TIP4P/F H[Formula see text]O, from PIMD simulations, is located at [Formula see text] MPa, [Formula see text] K, and [Formula see text] g/cm[Formula see text]. Isotope substitution effects are important; the LLCP location in q-TIP4P/F D[Formula see text]O is estimated to be [Formula see text] MPa, [Formula see text] K, and [Formula see text] g/cm[Formula see text]. Interestingly, for the water model studied, differences in the LLCP location from PIMD and MD simulations suggest that nuclear quantum effects (i.e., atoms delocalization) play an important role in the thermodynamics of water around the LLCP (from the MD simulations of q-TIP4P/F water, [Formula see text] MPa, [Formula see text] K, and [Formula see text] g/cm[Formula see text]). Overall, our results strongly support the LLPT scenario to explain water anomalous behavior, independently of the fundamental differences between classical MD and PIMD techniques. The reported values of [Formula see text] for D[Formula see text]O and, particularly, H[Formula see text]O suggest that improved water models are needed for the study of supercooled water.