Vibrational Density Of States Research Articles

Efficient Parametrization of Transferable Atomic Cluster Expansion for Water.

We present a highly accurate and transferable parametrization of water using the atomic cluster expansion (ACE). To efficiently sample liquid water, we propose a novel approach that involves sampling static calculations of various ice phases and utilizing the active learning (AL) feature of the ACE-based D-optimality algorithm to select relevant liquid water configurations, bypassing computationally intensive ab initio molecular dynamics simulations. Our results demonstrate that the ACE descriptors enable a potential initially fitted solely on ice structures, which is later upfitted with few configurations of liquid, identified with AL to provide an excellent description of liquid water. The developed potential exhibits remarkable agreement with first-principles reference, accurately capturing various properties of liquid water, including structural characteristics such as pair correlation functions, covalent bonding profiles, and hydrogen bonding profiles, as well as dynamic properties like the vibrational density of states, diffusion coefficient, and thermodynamic properties such as the melting point of the ice Ih. Our research introduces a new and efficient sampling technique for machine learning potentials in water simulations while also presenting a transferable interatomic potential for water that reveals the accuracy of first-principles reference. This advancement not only enhances our understanding of the relationship between ice and liquid water at the atomic level but also opens up new avenues for studying complex aqueous systems.

Model study on potential removal of toxic Se(VI) by organically modified montmorillonite

The theoretical DFT-D3 approach was used in the model study of the immobilisation of the toxic selenate oxyanion, with montmorillonite clay modified by poly(2-methyl-2-oxazoline) polymer represented by a pentamer unit, and tetrabutylphosphonium cation. The calculated basal spacing for Se-POx-Mt and Se-TBP-Mt models provided similar results differing by 0.2 Å (18.3 Å for Se-POx-Mt and/or 18.5 Å for Se-TBP-Mt, respectively). The calculated intercalation energy, ΔEint, showed a suitability of both modified Mts for trapping of (SeO4)2– oxyanions favouring the Se-TBP-Mt (–210.7 kJ/mol) system compared to Se-POx-Mt (–124.5 kJ/mol). The main interactions in both models were classified as hydrogen bonds of weak (CH···O) and moderate-to-strong (OH···O) strength. The calculated total and projected vibrational density of states of the Se-POx-Mt and Se‑TBP‑Mt models, obtained from the ab initio molecular dynamics simulations, were used for distinct identification of vibrational modes of the intercalated (SeO4)2– ion in the interlayer space of both models.

Molecular dynamics simulations of thermomechanical properties of silicone-modified phenolic polymer

The silicone-phenolic multicomponent polymers are typically employed as the matrix of fiber-reinforced nanocomposites developed for reentry vehicles due to their excellent thermal and mechanical properties. The thermomechanical properties of the silicone-phenolic multicomponent polymer system, which are greatly related to the processing and microstructures, were studied using molecular dynamics (MD) simulations combined with experiments. A multistep dynamic polymerization approach was utilized to form the crosslinked polymer model, which was also validated in terms of both microstructures and properties. The thermomechanical properties of the crosslinked polymer system were established as a function of crosslinking degree, component ratio, temperature, strain rate, and cooling rate, and the influence mechanisms of the processing parameters were revealed. The crosslinking degree can greatly influence the glass transition temperature and volumetric coefficient of thermal expansion, which is attributed to the constrained chain mobility. The crosslinking degree and the component ratio have a significant effect on the morphologies and vibrational density of states of the polymer system, respectively, which in turn affects the thermal conductivity. The failure mode during uniaxial tensile was investigated in terms of the atomic energy distribution through MD simulations. The elastic and plastic deformation stages are dominated by intermolecular non-bonding interactions, but less contributed by the bonding interactions. This work can guide the design of polymeric nanocomposites by establishing the relationship of processing-microstructure-thermomechanical properties.

Phonons in stringlet-land and the boson peak

Solid materials that deviate from the harmonic crystal paradigm exhibit characteristic anomalies in the specific heat and vibrational density of states (VDOS) with respect to Debye's theory predictions. The boson peak (BP), a low-frequency excess in the VDOS over Debye lawg(ω)∝ω2, is certainly the most famous among them; nevertheless, its origin is still subject of fierce debate. Recent simulation works provided strong evidence that localized one-dimensional string-like excitations (stringlets) might be the microscopic origin of the BP. In this work, we study the dynamics of acoustic phonons interacting with a bath of vibrating 1D stringlets with exponentially distributed size, as observed in simulations. We show that stringlets strongly renormalize the phonon propagator and naturally induce a BP anomaly in the VDOS, corresponding to the emergence of a dispersionless BP flat mode. Additionally, phonon-stringlet interactions produce a strong enhancement of sound attenuation and a dip in the speed of sound near the BP frequency, consistent with experimental and simulation data. The qualitative trends of the BP frequency and intensity are predicted within the model and shown to be in good agreement with previous observations. In summary, our results substantiate with a simple theoretical model the recent simulation results by Hu and Tanaka claiming the origin of the BP from stringlet dynamics.

Tailoring thermal transport and confinement effect of GaN/Si3N4 nanowires utilizing core–shell architecture

The thermal transport properties of nanowires (NWs) can be significantly influenced by the implementation of a core-shell structure, which introduces interface scattering and phonon localization effects, opening avenues for novel applications. In this paper, we use the method of non-equilibrium molecular dynamics to simulate the effects of system temperature, cross-sectional width, and nanopillar interface on the thermal transport of GaN/Si3N4core-shell NWs. The thermal transport process of phonons in core-shell NWs is studied by calculating the vibrational density of states, phonon participation rate, and dispersion curve. The results show that the core-shell NWs characterized by smooth interfaces exhibit a 17.4% decrease in thermal conductivity (TC) at room temperature when contrasted with pristine GaN NWs. Furthermore, the TC of GaN/Si3N4core-shell NWs can be further reduced by adding nanopillars at the interface. Due to resonance effect, thus effectively regulating the thermal transport. The presence of nanopillars increases phonon-surface scattering intensity at low-frequency and modifies phonon dispersion to decrease the group velocity. In addition, the hybridization of phonon modes between those of the nanopillars and the Si3N4shell gives rise to numerous dispersionless resonance phonon modes that span the entire phonon spectrum. This research delves into the effects of nanopillars and interfaces on thermal transport, providing important guidance for understanding confinement effects and establishing a robust theoretical basis for the regulation of thermal transport.

Effect of hydrogen constraints on predicting thermal conductivity of hydrocarbons in molecular dynamics simulation

Large overestimation has been reported while predicting the thermal conductivity of hydrocarbons using an all-atom force field model in molecular dynamics (MD) simulations. Although it has been guessed as resulting from the high-frequency vibration of the hydrogen atoms and hydrogen constraints method is suggested to be employed to reduce the deviation, how hydrogen constraints affect the heat conduction and local structure of hydrocarbons in MD simulation is still not fully understood. In this work, the effect of hydrogen constraints on the prediction of thermal conductivity of n-decane in MD simulations with the all-atom force field is studied. The results show that the deviation of the thermal conductivity of n-decane can be narrowed down by 72.48 % in simulations if the hydrogen constraints with a SHAKE algorithm is employed. The analysis of heat flux decomposition indicates that employing hydrogen constraints can reduce the contribution of transport term and non-bonded interactions to the heat flux, which in turn can help improve the accuracy while predicting the thermal conductivity in MD simulations. Furthermore, results of the vibrational density of states show that hydrogen constraints can dismiss the high-frequency vibration mode of molecules, thus effectively reducing the overestimation of thermal conductivity of hydrocarbon systems in MD simulations. The findings of this work sheds light on the molecular mechanism of heat transfer in hydrocarbon systems.

Irida-graphene phonon thermal transport via non-equilibrium molecular dynamics simulations.

Recently, a new 2D carbon allotrope called Irida-Graphene (Irida-G) was proposed, and its reliable stability has been previously predicted. Irida-G is a flat sheet topologically arranged into 3-6-8 carbon rings exhibiting metallic and non-magnetic properties. In this study, we investigated the thermal transport properties of Irida-G using classical reactive molecular dynamics simulations. The findings indicate that Irida-G has an intrinsic thermal conductivity of approximately 215 W mK-1 at room temperature, significantly lower than that of pristine graphene. This decrease is due to characteristic phonon scattering within Irida-G's porous structure. Additionally, the phonon group velocities and vibrational density of states for Irida-G were analyzed, revealing reduced average phonon group velocities compared to graphene. The thermal conductivity of Irida-G is isotropic and shows significant size effects, transitioning from ballistic to diffusive heat transport regimes as the system length increases. These results suggest that while Irida-G has lower thermal conductivity than graphene, it still holds potential for specific thermal management applications, sharing characteristics with other two-dimensional materials.

CO2 and NO2 formation on amorphous solid water

Context. The dynamics of molecule formation, relaxation, diffusion, and desorption on amorphous solid water (ASW) is studied in a quantitative fashion. Aims. The formation probability, stabilization, energy relaxation, and diffusion dynamics of CO2 and NO2 on cold ASW following atom+diatom recombination reactions are characterized quantitatively. Methods. Accurate machine-learned energy functions combined with fluctuating charge models were used to investigate the diffusion, interactions, and recombination dynamics of atomic oxygen with CO and NO on ASW. Energy relaxation to the ASW and into water internal degrees of freedom were determined from the analysis of the vibrational density of states. The surface diffusion and desorption energetics were investigated with extended and nonequilibrium MD simulations. Results. The reaction probability is determined quantitatively and it is demonstrated that surface diffusion of the reactants on the nanosecond time scale leads to recombination for initial separations of up to 20 Å. After recombination, both CO2 and NO2 stabilize by energy transfer to water internal and surface phonon modes on the picosecond timescale. The average diffusion barriers and desorption energies agree with those reported from experiments, which validates the energy functions. After recombination, the triatomic products diffuse easily, which contrasts with the equilibrium situation, in which both CO2 and NO2 are stationary on the multinanosecond timescale.

On-the-Fly Machine Learning Force Field Study of Liquid-Al/Solid-TiB2 Interfaces.

Using the on-the-fly machine learning force field, simulations were performed to study the atomic structure evolution of the liquid-Al/solid-TiB2 interface with two different terminations, aiming to deepen the understanding of the mechanism of TiB2 as nucleating particles in an aluminum alloy. We conducted simulations using MLFF for up to 100 ps, enabling us to observe the interfacial properties from a deeper and more comprehensive perspective. The nucleation potential of TiB2 particles is determined by the formation of various ordered structures at the interface, which is significantly influenced by the termination of the TiB2 (0001) surface. The evolution of the interface during heterogeneous nucleation processes with different terminations is described using structural information and dynamic characteristics. The Ti-terminated surface is more prone to forming quasi-solid regions compared to the B-termination. Analysis of mean square displacement and vibrational density of states indicates that the liquid layer at the Ti-terminated interface is closer in characteristics to a solid compared to the B-terminated interface. We also found that on the TiB2 (0001) surface different terminations give rise to distinct ordered structures at the interfaces, which is ascribed to their different diffusion abilities.

Machine-learned interatomic potentials for transition metal dichalcogenide Mo1−xWxS2−2ySe2y alloys

Machine Learned Interatomic Potentials (MLIPs) combine the predictive power of Density Functional Theory (DFT) with the speed and scaling of interatomic potentials, enabling theoretical spectroscopy to be applied to larger and more complex systems than is possible with DFT. In this work, we train an MLIP for quaternary Transition Metal Dichalcogenide (TMD) alloy systems of the form Mo1−xWxS2−2ySe2y, using the equivariant Neural Network (NN) MACE. We demonstrate the ability of this potential to calculate vibrational properties of alloy TMDs including phonon spectra for pure monolayers, and Vibrational Density of States (VDOS) and first-order Raman spectra for alloys across the range of x and y. We show that we retain DFT level accuracy while greatly extending feasible system size and extent of sampling over alloy configurations. We are able to characterize the first-order Raman active modes across the whole range of concentration, particularly for the “disorder-induced” modes.

Ultraviolet active novel chalcogenides AScSe2 (A= K, Cs): The structural, optoelectronic, mechanical, vibrational, and thermodynamical properties for energy harvesting applications

It is essential nowadays to investigate some unique and appropriate materials for optoelectronic applications. We deliberate here for the first time, the structural, optoelectronic, mechanical, vibrational, and thermodynamical properties of hexagonal structured selenium-based ternary chalcogenides AScSe2 (A = K, Cs) by using Perdew-Burke-Ernzerhof Generalized-Gradient-Approximation (PBE-GGA). The lattice angles for these materials are found as α = β = 90° and γ = 120°. KScSe2 is optimized with lattice parameters a = b = 4.3 (Å), c = 7.81 (Å) whereas, CsScSe2 is relaxed at a = b = 4.43 (Å) and c = 8.51 (Å). The hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional overestimates the lattice parameters to the extent that for KScSe2 these are found as a = b = 4.92 (Å), c = 7.10 (Å) and for CsScSe2 a = b = 5.15 (Å), c = 7.09 (Å). The energy band gap confirms the semiconducting nature of these materials. Born's criteria endorses the mechanical stability of these materials. Moreover, the temperature dependence of thermodynamic potentials and specific heat at constant volume is also determined using the harmonic approximation. The negative value of free energy ensures their thermodynamical stability. The vibrational modes are calculated by plotting the phonon dispersion and the vibrational density of states (VDOS) where infrared (IR) and Raman spectroscopy are used to characterize the vibrational modes. The optical parameters are examined at a smearing value of 0.5 eV that portray good absorptivity of the incident light from the Ultraviolet (UV) region. Thus, these materials may be potential ones to be utilized for energy harvesting applications via photovoltaic.

Effect of Li2O on the structure and properties of low‐boron aluminosilicate fiber glasses from molecular dynamics simulations and quantitative structure–property relationship analysis

AbstractMixed alkaline earth (MgO, CaO) aluminosilicate glass fibers (MCAS) with and without boron are commonly used as reinforcements in plastic composites, and the fundamental understanding of the thermal and mechanical properties is essential to the design of new glass compositions to satisfy the growing demands in applications from renewable energy to lightweight structural components. In this work, a series of Li2O containing low‐boron MCAS fiber glasses have been studied by using molecular dynamic (MD) simulations with recently developed effective partial charge composition dependent boron potentials. Structural characteristics, such as pair distribution function, bond angle distribution, and neutron/X‐ray diffraction structure factors, were calculated, as well as properties such as elastic moduli and vibrational density of states. The addition of Li2O was found to improve the elastic moduli of the fiber glasses in excellent agreement with experimental results we reported earlier. The simulation results showed that the weakened network connectivity and decrease of tri‐/bridging oxygen have positively affected the lowering of liquid temperature, owing to the transformation to more boron Q2 and silicon/aluminon Q3. It is found that higher oxygen packing density, coordinated aluminum/boron species such as [AlO5] and [BO4] units, larger‐membered oxide rings, and intensified connections of [AlOx] and [SiO4] are the main reasons that lead to improved mechanical properties. MD‐based quantitative structure–property relationship analyses were performed and showed excellent correlations to measured properties, indicating that it is a promising approach to understand glass properties and design new glass compositions for functional applications.

Modeling Vibrational Sum Frequency Generation Spectra of Interfacial Water on a Gold Surface: The Role of the Fermi Resonance.

Studying the hydrogen bonding structure of H2O at the metal-water interface is a highly complex yet fascinating endeavor. The intricate interactions and diverse orientations of water molecules on metal surfaces with varying potentials pose a significant challenge in elucidating the coupling between O-H stretching and H-O-H bending modes. In this study, we employed DFT-MD simulation to explore how the orientation of interfacial water molecules changes with the applied potential on the Au(111) surface. Based on the surface-specific velocity-velocity correlation function (ssVVCF) formula, we calculated vibrational sum frequency generation (VSFG) spectra for the O-H stretches. We found that three assigned peaks (∼3300, ∼3450, and 3650 cm-1) shifted toward lower frequencies as the potential moved toward more negative values. Our results align remarkably well with experimental Raman spectroscopy data. Notably, our VSFG analysis revealed a significant change in the VSFG spectra of the hydrogen-bonded O-H groups (∼3300 cm-1), switching from a negative to a positive sign with decreasing potential. This alteration suggests a substantial change in the orientation of these low-frequency O-H groups owing to their increased interactions with the Au surface. In contrast, the orientations of both the high-frequency O-H groups (∼3450 cm-1) and the dangling O-H groups (∼3650 cm-1) remained unaffected by the applied potentials. Furthermore, our analysis of the decomposed vibrational density of states (VDOS) for the H-O-H bending mode uncovered the coupling between the H-O-H bending and O-H stretching vibrations, known as the Fermi resonance. Our work suggests that the H-O-H bending vibration becomes restricted when water molecules transition from the ″one-H-down″ to the ″two-H-down″ conformation, leading to a redshift in the O-H stretching vibration through the Fermi resonance. By constructing the VSFG and decomposed VDOS spectra, we gained valuable insights into the structural changes that Raman spectra alone cannot fully interpret. Specifically, our analysis revealed the critical role of the Fermi resonance effect in shaping the spectroscopic signature of interfacial water molecules on the Au(111) surface.

Role of elastic phonon couplings in dictating the thermal transport across atomically sharp SiC/Si interfaces

The interfaces between SiC and the corresponding substrate largely affect the performance of SiC-based electronics. Understanding and designing the interfacial thermal transport across the SiC/substrate interfaces is critical for the thermal management design of these SiC-based power electronics. In this work, we systematically investigate the heat transfer across the 3C-SiC/Si, 4H-SiC/Si, and 6H-SiC/Si interfaces using non-equilibrium molecular dynamics simulations and diffuse mismatch model. We find that the room temperature ITC for 3C-SiC/Si, 4H-SiC/Si, and 6H-SiC/Si interfaces is 932 MW/m2K, 759 MW/m2K, and 697 MW/m2K, respectively, which is among the highest values for all interfaces made up of semiconductors (Yue et al., 2011; Cheng et al., 2020; Wilson et al., 2015; Ziade et al., 2015) [1–4]. The ultrahigh ITC of SiC/Si heterointerfaces at room temperature and high temperatures results from the dictating elastic scatterings at interfaces. We further find the ITC contributed by the elastic scattering decreases with the temperature but remains at a high ratio of 67%-78% even at an ultrahigh temperature of 1000 K. The reason for such a high elastic ITC is the large overlap between the vibrational density of states of Si and SiC at low and middle frequencies (<∼18 THz), which is also demonstrated by the diffuse mismatch model.

Enhanced heat transfer and thermal storage performance of molten K2CO3 by ZnO nanoparticles: A molecular dynamics study

Molten salt serves as a popular medium for thermal storage and transfer in concentrated solar power (CSP) systems. The existing experimental studies have found the capacity of enhancing the heat storage property of the molten carbonate by doping ZnO nanoparticles. However, it has been rarely reported to conduct simulations of ZnO/molten carbonate nanofluids, and the underlying mechanism of how ZnO nanoparticles enhance the heat transfer and storage capacity has less been explored. In this study, molecular dynamics (MD) simulation has been used to investigate the heat transfer and storage properties of a molten K2CO3 nanofluid containing ZnO nanoparticles. It was found that as the nanoparticle mass ratio in molten salt based nanofluids rises, their specific heat capacity initially increases but subsequently diminishes. The specific heat capacity peaks when the ZnO mass ratio is at 1 wt%, resulting in an increase of 17.83 % compared with the base salt. The thermal conductivity in nanofluids based on molten salt increases with the increasing of ZnO mass ratio. Multi-perspective analysis of the mechanism how ZnO nanoparticles enhance the thermal energy storage performance of molten salt, including the microstructural analysis, number density, vibrational density of state, and heat flow decomposition, is conducted. In addition, results of the number density illustrates that a compressed layer of K+ is formed surrounding the ZnO nanoparticles, and the change in the potential energy leads to an increasing heat capacity. Analysis of the heat flow decomposition suggests that the enhanced nonbonded interaction is the main reason for the improvement of thermal conductive performance in the nanofluids based on molten salt. The findings in the present study provide a new perspective of how ZnO nanoparticles enhance the thermophysical properties of molten salts.

Stringlet excitation model of the boson peak.

The boson peak (BP), a low-energy excess in the vibrational density of states over the Debye contribution, is often identified as a characteristic of amorphous solid materials. Despite decades of efforts, its microscopic origin still remains a mystery. Recently, it has been proposed, and corroborated with simulations, that the BP might stem from intrinsic localized modes involving one-dimensional (1D) string-like excitations ("stringlets"). We build on a theory originally proposed by Lund that describes the localized modes as 1D vibrating strings, but we specify the stringlet size distribution to be exponential, as observed in simulations. We provide an analytical prediction for the BP frequency ωBP in the temperature regime well below the observed glass transition temperature Tg. The prediction involves no free parameters and accords quantitatively with prior simulation observations in 2D and 3D model glasses based on inverse power law potentials. The comparison of the string model to observations is more uncertain when compared to simulations of an Al-Sm metallic glass material at temperatures well above Tg. Nonetheless, our stringlet model of the BP naturally reproduces the softening of the BP frequency upon heating and offers an analytical explanation for the experimentally observed scaling with the shear modulus in the glass state and changes in this scaling in simulations of glass-forming liquids. Finally, the theoretical analysis highlights the existence of a strong damping for the stringlet modes above Tg, which leads to a large low-frequency contribution to the 3D vibrational density of states, observed in both experiments and simulations.

Insight into Interfacial Heat Transfer of β-Ga2O3/Diamond Heterostructures via the Machine Learning Potential.

β-Ga2O3 is an ultrawide-band gap semiconductor with excellent potential for high-power and ultraviolet optoelectronic device applications. Low thermal conductivity is one of the major obstacles to enable the full performance of β-Ga2O3-based devices. A promising solution for this problem is to integrate β-Ga2O3 with a diamond heat sink. However, the thermal properties of the β-Ga2O3/diamond heterostructures after the interfacial bonding have not been studied extensively, which are influenced by the crystal orientations and interfacial atoms for the β-Ga2O3 and diamond interfaces. In this work, molecular dynamics simulations based on machine learning potential have been adopted to investigate the crystal-orientation-dependent and interfacial-atom-dependent thermal boundary resistance (TBR) of the β-Ga2O3/diamond heterostructure after interfacial bonding. The differences in TBR at different interfaces are explained in detail through the explorations of thermal conductivity value, thermal conductivity spectra, vibration density of states, and interfacial structures. Based on the above explorations, a further understanding of the influence of different crystal orientations and interfacial atoms on the β-Ga2O3/diamond heterostructure was achieved. Finally, insightful optimization strategies have been proposed in the study, which could pave the way for better thermal design and management of β-Ga2O3/diamond heterostructures according to guidance in the selection of the crystal orientations and interfacial atoms of the β-Ga2O3 and diamond interfaces.

Direct prediction of isotopic properties from molecular dynamics trajectories: Application to sulfide, sulfate and sulfur radical ions in hydrothermal fluids

In hydrothermal fluids, disulfur (S2•−) and trisulfur (S3•−) radical anions have been observed to coexist with the major hydrogen sulfide and sulfate species. These radical ions have potentially important effects on the solubility, transport and fractionation of metals and sulfur, with consequences for ore deposit formation and, more generally, for geochemical cycles of metals and volatiles. It is therefore essential to know the intrinsic isotopic properties of these important sulfur species in order to use sulfur isotopes for tracing different geological processes. Here, the theoretical equilibrium isotopic properties of the disulfur and trisulfur radical ions are computed and compared to the hydrogen sulfide (H2S) and sulfate ion (SO42−), using, for the first time, a first-principles molecular dynamics (MD) approach. The isotopic properties are calculated directly from molecular dynamics trajectories using the vibrational density of states and the atomic kinetic energy, and then compared to the more established method based on sampling of several snapshots. This comparison allowed us to validate the new modelling method and to assess its advantages and limitations. The predicted equilibrium isotope fractionation in terms of 34S/32S between S2•− and S3•− is small, i.e. <1‰, with a slight enrichment in the heavier isotope for S3•−, over the temperature range 200–500 °C. Both radical ions are slightly depleted in the heavier isotope, by 1 to 2‰, relative to aqueous H2S. Our results help tuning sulfur isotope fractionation models used for tracing the origin and evolution of hydrothermal fluids. Our method opens large perspectives for using the rapidly growing body of MD simulation data in geosciences on structure and stability of aqueous complexes to assess in parallel element isotope fractionations from MD-generated trajectories.

Superdiffusive Rotation of Interfacial Water on Noble Metal Surface.

Interfacial water on a metal surface acts as an active layer through the reorientation of water, thereby facilitating the energy transfer and chemical reaction across the metal surface in various physicochemical and industrial processes. However, how this active interfacial water collectively behaves on flat noble metal substrates remains largely unknown due to the experimental limitation in capturing librational vibrational motion of interfacial water and prohibitive computational costs at the first-principles level. Herein, by implementing a machine-learning approach to train neural network potentials, we enable performing advanced molecular dynamics simulations with ab initio accuracy at a nanosecond scale to map the distinct rotational motion of water molecules on a metal surface at room temperature. The vibrational density of states of the interfacial water with two-layer profiles reveals that the rotation and vibration of water within the strong adsorption layer on the metal surface behave as if the water molecules in the bulk ice, wherein the O-H stretching frequency is well consistent with the experimental results. Unexpectedly, the water molecules within the adjacent weak adsorption layer exhibit superdiffusive rotation, contrary to the conventional diffusive rotation of bulk water, while the vibrational motion maintains the characteristic of bulk water. The mechanism underlying this abnormal superdiffusive rotation is attributed to the translation-rotation decoupling of water, in which the translation is restrained by the strong hydrogen bonding within the bilayer interfacial water, whereas the rotation is accelerated freely by the asymmetric water environment. This superdiffusive rotation dynamics may elucidate the experimentally observed large fluctuation of the potential of zero charge on Pt and thereby the conventional Helmholtz layer model revised by including the contribution of interfacial water orientation. The surprising superdiffusive rotation of vicinal water next to noble metals will shed new light on the physicochemical processes and the activity of water molecules near metal electrodes or catalysts.

Elasticity of self-organized frustrated disordered spring networks.

There have been some interesting recent advances in understanding the notion of mechanical disorder in structural glasses and the statistical mechanics of these systems' low-energy excitations. Here we contribute to these advances by studying a minimal model for structural glasses' elasticity in which the degree of mechanical disorder-as characterized by recently introduced dimensionless quantifiers-is readily tunable over a very large range. We comprehensively investigate a number of scaling laws observed for various macro, meso and microscopic elastic properties, and rationalize them using scaling arguments. Interestingly, we demonstrate that the model features the universal quartic glassy vibrational density of states as seen in many atomistic and molecular models of structural glasses formed by cooling a melt. The emergence of this universal glassy spectrum highlights the role of self-organization (toward mechanical equilibrium) in its formation, and elucidates why models featuring structural frustration alone do not feature the same universal glassy spectrum. Finally, we discuss relations to existing work in the context of strain stiffening of elastic networks and of low-energy excitations in structural glasses, in addition to future research directions.