Nuclear Quantum Effects on the Thermodynamic, Structural, and Dynamical Properties of Equilibrium and Supercooled Water

We perform path-integral molecular dynamics (PIMD) simulations of H₂O and D₂O using the qTIP4P/F model. Simulations are performed at P = 1 bar and for temperatures 200 < T < 375 K. The density of H₂O and D₂O calculated from PIMD simulations are in excellent agreement with experiments at T > 230 K. We also evaluated the thermal expansion coefficient α_p(T), isothermal compressibility κ_T(T), isobaric heat capacity C_P(T), and the dielectric permittivity ε(T). We found that at T > 273 K, α_P(T) and κ_T(T) are in perfect agreement with experiments while C_P(T) and ε(T) are in semi-quantitative agreement. However, deviations between PIMD simulations and experiments, become more pronounced upon supercooling implying that the inclusion of nuclear quantum effects in PIMD simulations of qTIP4P/F water are not sufficient to reproduce the large fluctuations in density and entropy characteristic of supercooled water. We also calculated the diffusion coefficient of H₂O and D₂O using the ring-polymer molecular dynamics (RPMD) approach and find that computer simulations are in good agreement with experiments at all temperatures studied. Our results from PIMD simulations are not inconsistent with the possibility that H₂O and D₂O exhibit a liquid-liquid critical point at low temperature.