 NSFPAR ID:
 10284028
 Date Published:
 Journal Name:
 Frontiers in Chemistry
 Volume:
 9
 ISSN:
 22962646
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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We perform path integral molecular dynamics (PIMD) simulations of a monatomic liquid that exhibits a liquid–liquid phase transition and liquid–liquid critical point. PIMD simulations are performed using different values of Planck’s constant h, allowing us to study the behavior of the liquid as nuclear quantum effects (NQE, i.e., atoms delocalization) are introduced, from the classical liquid ( h = 0) to increasingly quantum liquids ( h > 0). By combining the PIMD simulations with the ringpolymer molecular dynamics method, we also explore the dynamics of the classical and quantum liquids. We find that (i) the glass transition temperature of the lowdensity liquid (LDL) is anomalous, i.e., [Formula: see text] decreases upon compression. Instead, (ii) the glass transition temperature of the highdensity liquid (HDL) is normal, i.e., [Formula: see text] increases upon compression. (iii) NQE shift both [Formula: see text] and [Formula: see text] toward lower temperatures, but NQE are more pronounced on HDL. We also study the glass behavior of the ringpolymer systems associated with the quantum liquids studied (via the pathintegral formulation of statistical mechanics). There are two glass states in all the systems studied, lowdensity amorphous ice (LDA) and highdensity amorphous ice (HDA), which are the glass counterparts of LDL and HDL. In all cases, the pressureinduced LDA–HDA transformation is sharp, reminiscent of a firstorder phase transition. In the lowquantum regime, the LDA–HDA transformation is reversible, with identical LDA forms before compression and after decompression. However, in the highquantum regime, the atoms become more delocalized in the final LDA than in the initial LDA, raising questions on the reversibility of the LDA–HDA transformation.more » « less

Abstract We perform pathintegral molecular dynamics (PIMD), ringpolymer MD (RPMD), and classical MD simulations of H
O and D$$_2$$ ${}_{2}$ O using the qTIP4P/F water model over a wide range of temperatures and pressures. The density$$_2$$ ${}_{2}$ , isothermal compressibility$$\rho (T)$$ $\rho \left(T\right)$ , and selfdiffusion coefficients$$\kappa _T(T)$$ ${\kappa}_{T}\left(T\right)$D (T ) of H O and D$$_2$$ ${}_{2}$ O are in excellent agreement with available experimental data; the isobaric heat capacity$$_2$$ ${}_{2}$ 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$$C_P(T)$$ ${C}_{P}\left(T\right)$ O and D$$_2$$ ${}_{2}$ O exhibit a liquidliquid 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 TwoStateEquationofState (TSEOS). Using the TSEOS, we estimate that the LLCP for qTIP4P/F H$$_2$$ ${}_{2}$ O, from PIMD simulations, is located at$$_2$$ ${}_{2}$ MPa,$$P_c = 167 \pm 9$$ ${P}_{c}=167\pm 9$ K, and$$T_c = 159 \pm 6$$ ${T}_{c}=159\pm 6$ g/cm$$\rho _c = 1.02 \pm 0.01$$ ${\rho}_{c}=1.02\pm 0.01$ . Isotope substitution effects are important; the LLCP location in qTIP4P/F D$$^3$$ ${}^{3}$ O is estimated to be$$_2$$ ${}_{2}$ MPa,$$P_c = 176 \pm 4$$ ${P}_{c}=176\pm 4$ K, and$$T_c = 177 \pm 2$$ ${T}_{c}=177\pm 2$ g/cm$$\rho _c = 1.13 \pm 0.01$$ ${\rho}_{c}=1.13\pm 0.01$ . 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 qTIP4P/F water,$$^3$$ ${}^{3}$ MPa,$$P_c = 203 \pm 4$$ ${P}_{c}=203\pm 4$ K, and$$T_c = 175 \pm 2$$ ${T}_{c}=175\pm 2$ g/cm$$\rho _c = 1.03 \pm 0.01$$ ${\rho}_{c}=1.03\pm 0.01$ ). 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$$^3$$ ${}^{3}$ for D$$T_c$$ ${T}_{c}$ O and, particularly, H$$_2$$ ${}_{2}$ O suggest that improved water models are needed for the study of supercooled water.$$_2$$ ${}_{2}$ 
Abstract Some of the most exotic properties of the quantum vacuum are predicted in ultrastrongly coupled photon–atom systems; one such property is quantum squeezing leading to suppressed quantum fluctuations of photons and atoms. This squeezing is unique because (1) it is realized in the ground state of the system and does not require external driving, and (2) the squeezing can be perfect in the sense that quantum fluctuations of certain observables are completely suppressed. Specifically, we investigate the ground state of the Dicke model, which describes atoms collectively coupled to a single photonic mode, and we found that the photon–atom fluctuation vanishes at the onset of the superradiant phase transition in the thermodynamic limit of an infinite number of atoms. Moreover, when a finite number of atoms is considered, the variance of the fluctuation around the critical point asymptotically converges to zero, as the number of atoms is increased. In contrast to the squeezed states of flying photons obtained using standard generation protocols with external driving, the squeezing obtained in the ground state of the ultrastrongly coupled photon–atom systems is resilient against unpredictable noise.

We present a bottomup coarsegraining (CG) method to establish implicitsolvent CG modeling for polymers in solution, which conserves the dynamic properties of the reference microscopic system. In particular, tens to hundreds of bonded polymer atoms (or LennardJones beads) are coarsegrained as one CG particle, and the solvent degrees of freedom are eliminated. The dynamics of the CG system is governed by the generalized Langevin equation (GLE) derived via the MoriZwanzig formalism, by which the CG variables can be directly and rigorously linked to the microscopic dynamics generated by molecular dynamics (MD) simulations. The solventmediated dynamics of polymers is modeled by the nonMarkovian stochastic dynamics in GLE, where the memory kernel can be computed from the MD trajectories. To circumvent the difficulty in direct evaluation of the memory term and generation of colored noise, we exploit the equivalence between the nonMarkovian dynamics and Markovian dynamics in an extended space. To this end, the CG system is supplemented with auxiliary variables that are coupled linearly to the momentum and among themselves, subject to uncorrelated Gaussian white noise. A highorder timeintegration scheme is used to solve the extended dynamics to further accelerate the CG simulations. To assess, validate, and demonstrate the established implicitsolvent CG modeling, we have applied it to study four different types of polymers in solution. The dynamic properties of polymers characterized by the velocity autocorrelation function, diffusion coefficient, and mean square displacement as functions of time are evaluated in both CG and MD simulations. Results show that the extended dynamics with auxiliary variables can construct arbitrarily highorder CG models to reproduce dynamic properties of the reference microscopic system and to characterize longtime dynamics of polymers in solution.more » « less

Ab initio molecular dynamics liquidquench simulations and hybrid density functional calculations are performed to model the effects of roomtemperature atomic fluctuations and photoillumination on the structural and electronic properties of amorphous substoichiometric In2O2.96. A large configurational ensemble is employed to reliably predict the distribution of localized defects as well as their response to the thermal and light activation. The results reveal that the illumination effects on the carrier concentration are greater in amorphous configurations with shorter In–O bond length and reduced polyhedral sharing as compared to the structures with a more uniform morphology. The obtained correlation between the photoinduced carrier density and the reduction in the number of fully coordinated Inatoms implies that metal oxides with a significant fraction of crystalline/amorphous interfaces would show a more pronounced response to illumination. Photoexcitation also produces In–O2–In defects that have not been previously found in substoichiometric amorphous oxides; these defects are responsible for carrier instabilities due to overdoping.