Nuclear quantum effects on the thermodynamic, structural, and dynamical properties of water.

We perform path-integral molecular dynamics (PIMD) simulations of HO and DO using the q-TIP4P/F model. Simulations are performed at P = 1 bar and over a wide range of temperatures that include the equilibrium (T≥ 273 K) and supercooled (210 ≤T < 273 K) liquid states of water. The densities of both HO and DO calculated from PIMD simulations are in excellent agreement with experiments in the equilibrium and supercooled regimes. We also evaluate important thermodynamic response functions, specifically, the thermal expansion coefficient α(T), isothermal compressibility κ(T), isobaric heat capacity C(T), and static dielectric constant ε(T). While these properties are in excellent [α(T) and κ(T)] or semi-quantitative agreement [C(T) and ε(T)] with experiments in the equilibrium regime, they are increasingly underestimated upon further cooling. It follows that the inclusion of nuclear quantum effects in PIMD simulations of (q-TIP4P/F) water is not sufficient to reproduce the anomalous large fluctuations in density, entropy, and electric dipole moment characteristic of supercooled water. It has been hypothesized that water may exhibit a liquid-liquid critical point (LLCP) in the supercooled regime at P > 1 bar and that such a LLCP generates a maximum in C(T) and κ(T) at 1 bar. Consistent with this hypothesis and in particular, with experiments, we find a maximum in the κ(T) of q-TIP4P/F light and heavy water at T≈ 230-235 K. No maximum in C(T) could be detected down to T≥ 210 K. We also calculate the diffusion coefficient D(T) of HO and DO using the ring-polymer molecular dynamics (RPMD) technique and find that computer simulations are in remarkable good agreement with experiments at all temperatures studied. The results from RPMD/PIMD simulations are also compared with the corresponding results obtained from classical MD simulations of q-TIP4P/F water where atoms are represented by single interacting sites. Surprisingly, we find minor differences in most of the properties studied, with C(T), D(T), and structural properties being the only (expected) exceptions.