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Abstract We perform path-integral molecular dynamics (PIMD), ring-polymer MD (RPMD), and classical MD simulations of H$$_2$$ O and D$$_2$$ O using the q-TIP4P/F water model over a wide range of temperatures and pressures. The density$$\rho (T)$$ , isothermal compressibility$$\kappa _T(T)$$ , and self-diffusion coefficientsD(T) of H$$_2$$ O and D$$_2$$ O are in excellent agreement with available experimental data; the isobaric heat capacity$$C_P(T)$$ 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$$_2$$ O and D$$_2$$ O exhibit a liquid-liquid critical point (LLCP) at low temperatures and positive pressures. The data from PIMD/MD for H$$_2$$ O and D$$_2$$ 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$$_2$$ O, from PIMD simulations, is located at$$P_c = 167 \pm 9$$ MPa,$$T_c = 159 \pm 6$$ K, and$$\rho _c = 1.02 \pm 0.01$$ g/cm$$^3$$ . Isotope substitution effects are important; the LLCP location in q-TIP4P/F D$$_2$$ O is estimated to be$$P_c = 176 \pm 4$$ MPa,$$T_c = 177 \pm 2$$ K, and$$\rho _c = 1.13 \pm 0.01$$ g/cm$$^3$$ . 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,$$P_c = 203 \pm 4$$ MPa,$$T_c = 175 \pm 2$$ K, and$$\rho _c = 1.03 \pm 0.01$$ g/cm$$^3$$ ). 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$$T_c$$ for D$$_2$$ O and, particularly, H$$_2$$ O suggest that improved water models are needed for the study of supercooled water.
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Positron pair interactions in a nearly free-electron metal
AbstractThe electric field surrounding a single positron in a metal is screened by an increase in the local electron density which, in the case of nearly free-electron metals (like Al, Na, etc.), has a radial distribution similar to that of the electron in positronium (Ps). In such metals, a singlet pair of positrons would experience an attractive interaction and at low enough electron densities could possibly form a bound state that is held together by exchange and correlation energies, thus forming structures analogous to that of the positronium molecule (Ps$$_2$$ ), with binding energies of a few tenths of an eV. Such di-positrons could be prevalent at positron densities of around 10$$^{18}$$ cm$$^{-3}$$ and, if so, would be evident from an apparent broadening of the sharp step at the Fermi surface in measurements of the electron momentum distribution by the angular correlation of the 2$$\gamma $$ annihilation radiation. Even if di-positrons are not directly formed in a metal, optical spectroscopy of Ps$$_2$$ formed in vacuum via pairs of positrons simultaneously being emitted from the surface could be applied to the direct measurement of the momentum distribution of Cooper pairs. If they exist, di-positrons in metals would yield interesting information about electron and positron interactions and at very high densities might allow the study of a di-positron Bose–Einstein condensate immersed in an electron gas. Graphic Abstract
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- Award ID(s):
- 2011836
- PAR ID:
- 10369241
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- The European Physical Journal D
- Volume:
- 76
- Issue:
- 6
- ISSN:
- 1434-6060
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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