Evidence of a liquid–liquid phase transition in H$$_2$$O and D$$_2$$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$$_2$$${}_{2}$O and D$$_2$$${}_{2}$O using the q-TIP4P/F water model over a wide range of temperatures and pressures. The density$$\rho (T)$$$\rho \left(T\right)$, isothermal compressibility$$\kappa _T(T)$$${\kappa }_{T}\left(T\right)$, and self-diffusion coefficientsD(T) of H$$_2$$${}_{2}$O and D$$_2$$${}_{2}$O are in excellent agreement with available experimental data; the isobaric heat capacity$$C_P(T)$$${C}_{P}\left(T\right)$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$$_2$$${}_{2}$O and D$$_2$$${}_{2}$O exhibit a liquid-liquid critical point (LLCP) at low temperatures and positive pressures. The data from PIMD/MD for H$$_2$$${}_{2}$O and D$$_2$$${}_{2}$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$$_2$$${}_{2}$O, from PIMD simulations, is located at$$P_c = 167 \pm 9$$${P}_{c}=167±9$ MPa,$$T_c = 159 \pm 6$$${T}_{c}=159±6$ K, and$$\rho _c = 1.02 \pm 0.01$$${\rho }_{c}=1.02±0.01$ g/cm$$^3$$${}^{3}$. Isotope substitution effects are important; the LLCP location in q-TIP4P/F D$$_2$$${}_{2}$O is estimated to be$$P_c = 176 \pm 4$$${P}_{c}=176±4$ MPa,$$T_c = 177 \pm 2$$${T}_{c}=177±2$ K, and$$\rho _c = 1.13 \pm 0.01$$${\rho }_{c}=1.13±0.01$ g/cm$$^3$$${}^{3}$. Interestingly, for the water model studied, differences in the LLCP location from PIMD and MD simulations suggest that nuclear quantum effects more »

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Award ID(s):
Publication Date:
NSF-PAR ID:
10364848
Journal Name:
Scientific Reports
Volume:
12
Issue:
1
ISSN:
2045-2322
Publisher:
Nature Publishing Group
National Science Foundation
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5. Abstract

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