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

We perform path-integral molecular dynamics (PIMD) simulations of H2O and D2O 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 H2O and D2O 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 alpha(P)(T), isothermal compressibility kappa(T)(T), isobaric heat capacity C-P(T), and static dielectric constant epsilon(T). While these properties are in excellent [alpha(P)(T) and kappa(T)(T)] or semi-quantitative agreement [C-P(T) and epsilon(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-P(T) and kappa(T)(T) at 1 bar. Consistent with this hypothesis and in particular, with experiments, we find a maximum in the kappa(T)(T) of q-TIP4P/F light and heavy water at T approximate to 230-235 K. No maximum in C-P(T) could be detected down to T >= 210 K. We also calculate the diffusion coefficient D(T) of H2O and D2O 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-P(T), D(T), and structural properties being the only (expected) exceptions.

Automated summary

PIMD and RPMD simulations were used to study the properties of water and heavy water, revealing limitations in replicating the observed fluctuations in properties under supercooled conditions. Comparison with classical MD simulations showed minor differences in most properties, with exceptions in C-P(T), D(T), and structural properties.