Nonequilibrium Ab Initio Molecular Dynamics Research Articles

Diffusivity of individual adsorbed water molecules at an Fe2O3 − Hematite/Water interface under external electric-field conditions

The dynamical properties of physically and chemically adsorbed water molecules at a pristine hematite-(001) surfaces have been studied by non-equilibrium ab-initio molecular dynamics in the constant-volume, constant-temperature ensemble at room temperature, under externally-applied, uniform static electric fields of increasing intensity. Significant alterations in the dipole moment and self-diffusion constant were found vis-à-vis zero-field conditions, as well as shifting of the probability distribution of individual molecular self-diffusivities. For instance, application of static fields was found to increase the self-diffusion of water molecules at the α-hematite Fe2O3 surface, effectively due to some extent of ‘dipole-locking’ in the direction of the applied field.

Nonequilibrium Ab Initio Molecular Dynamics Simulation of Water Splitting at Fe2O3-Hematite/Water Interfaces in an External Electric Field.

In the exploration of the optimal material for achieving the photoelectrochemical dissociation of water into hydrogen, hematite (α-Fe2O3) emerges as a highly promising candidate for proof-of-concept demonstrations. Recent studies suggest that the concurrent application of external electric fields could enhance the photoelectrochemical (PEC) process. To delve into this, we conducted nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations in this study, focusing on hematite-water interfaces at room temperature under progressively stronger electric fields. Our findings reveal intriguing evidence of water molecule adsorption and dissociation, as evidenced by an analysis of the structural properties of the hydrated layered surface of the hematite-water interface. Additionally, we scrutinized intermolecular structures using radial distribution functions (RDFs) to explore the interaction between the hematite slab and water. Notably, the presence of a Grotthuss hopping mechanism became apparent as the electric field strength increased. A comprehensive discussion based on intramolecular geometry highlighted aspects such as hydrogen-bond lengths, H-bond angles, average H-bond numbers, and the observed correlation existing among the hydrogen-bond strength, bond-dissociation energy, and H-bond lifetime. Furthermore, we assessed the impact of electric fields on the librational, bending, and stretching modes of hydrogen atoms in water by calculating the vibrational density of states (VDOS). This analysis revealed distinct field effects for the three characteristic band modes, both in the bulk region and at the hematite-water interface. We also evaluated the charge density of active elements at the aqueous hematite surface, delving into field-induced electronic charge-density variations through the Hirshfeld charge density analysis of atomic elements. Throughout this work, we drew clear distinctions between parallel and antiparallel field alignments at the hematite-water interface, aiming to elucidate crucial differences in local behavior for each surface direction of the hematite-water interface.

Energy relaxation pathways and their isotope effects of water bending mode in liquid phase: A nonequilibrium ab initio molecular dynamics simulation study

Vibrational energy relaxation pathways and the rates of excited bending mode of water molecules (H2O) in bulk liquid water (H2O), and the rates of the excited bending mode of HOD molecules in D2O (isotopically diluted water) were examined via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulation. The NE-AIMD simulation showed a significantly fast relaxation of the bend vibration, at the timescale of 127 fs in pure water and 144 fs in isotopically diluted water. The main relaxation pathway for pure and isotopically diluted water was found to be driven by intramolecular bend–libration coupling (84% in pure water and 96% in isotopically diluted water). In the case of pure water, intermolecular coupling between bend–bend vibrations was small but non-negligible (11%), whereas such intermolecular coupling was negligible in isotopically diluted water owing to a large frequency mismatch between the excited bend vibration and the surrounding bend vibrations.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface.

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T1 = 0.13 ps, which is slightly larger than that in the bulk (T1 = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T1 = 0.74 ps in the bulk decreases to T1 = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Water Breakup at Fe2O3-Hematite/Water Interfaces: Influence of External Electric Fields from Nonequilibrium Ab Initio Molecular Dynamics.

The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The dissociation of water molecules to form chemically adsorbed species was scrutinized, in addition to charge redistribution and Grotthus proton hopping between water molecules. Dynamical properties of the adsorbed water molecules and OH– and H3O+ ions were gauged, such as the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. The development of Helmholtz charge layers via water breakup at Fe2O3–hematite/water interfaces is also an interesting feature, with the development of protonic conduction on the surface and more bulk-like water.

Electric-field-promoted photo-electrochemical production of hydrogen from water splitting

Given that conversion efficiencies of incident solar radiation to liquid fuels, e.g., H2, are of the order of a few percent or less, as quantified by ‘solar to hydrogen’ (STH), economically inexpensive and operationally straightforward ways to boost photo-electrochemcial (PEC) H2 production from solar-driven water splitting are important. In this work, externally-applied static electric fields have led to enhanced H2 production in an energy-efficient manner, with up to ~30–40% increase in H2 (bearing in mind field-input energy) in a prototype, open-type solar cell featuring rutile/titania and hematite/iron-oxide (Fe2O3), respectively, in contact with an alkaline aqueous medium (corresponding to respective relative increases of STH by ~12 and 16%). We have also performed non-equilibrium ab-initio molecular dynamics in both static electric and electromagnetic (e/m) fields, for water in contact with a hematite/iron-oxide (001) surface, observing enhanced break-up of water molecules, by up to ~70% in the linear-response régime. We discuss the microscopic origin of such enhanced water-splitting, based on experimental and simulation-based insights. In particular, we external-field direction at the hematite surfaces, and scrutinise properties of the adsorbed water molecules and OH– and H3O+ species, e.g., hydrogen bonds between water-protons and the hematite surfaces’ bridging oxygen atoms, as well as interactions between oxygen atoms in adsorbed water molecules and underlying iron atoms.

Ab initio molecular dynamics study on energy relaxation path of hydrogen-bonded OH vibration in bulk water.

The vibrational energy relaxation paths of hydrogen-bonded (H-bonded) OH excited in pure water and in isotopically diluted (deuterated) water are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The present study extends the previous NE-AIMD simulation for the energy relaxation of an excited free OH vibration at an air/water interface [T. Ishiyama, J. Chem. Phys. 154, 104708 (2021)] to the energy relaxation of an excited H-bonded OH vibration in bulk water. The present simulation shows that the excited OH vibration in pure water dissipates its energy on a timescale of 0.1ps, whereas that in deuterated water relaxes on a timescale of 0.7ps, consistent with the experimental observations. To decompose these relaxation energies into the components due to intramolecular and intermolecular couplings, constraints are introduced on the vibrational modes except for the target path in the NE-AIMD simulation. In the case of pure water, 80% of the total relaxation is attributed to the pathway due to the resonant intermolecular OH⋯OH stretch coupling, and the remaining 17% and 3% are attributed to intramolecular couplings with the bend overtone and with the conjugate OH stretch, respectively. This result strongly supports a significant role for the Förster transfer mechanism of pure water due to the intermolecular dipole-dipole interactions. In the case of deuterated water, on the other hand, 36% of the total relaxation is due to the intermolecular stretch coupling, and all the remaining 64% arises from coupling with the intramolecular bend overtone.

Energy relaxation path of excited free OH vibration at an air/water interface revealed by nonequilibrium ab initio molecular dynamics simulation.

Nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations are conducted at an air/water interface to elucidate the vibrational energy relaxation path of excited non-hydrogen-bonded (free) OH. A recent time-resolved vibrational sum frequency generation (TR-VSFG) spectroscopy experiment revealed that the relaxation time scales of free OH at the surface of pure water and isotopically diluted water are very similar to each other. In the present study, the dynamics of free OH excited at the surface of pure water and deuterated water are examined with an NE-AIMD simulation, which reproduces the experimentally observed features. The relaxation paths are examined by introducing constraints for the bonds and angles of water molecules relevant to specific vibrational modes in NE-AIMD simulations. In the case of free OH relaxation at the pure water surface, stretching vibrational coupling with the conjugate bond makes a significant contribution to the relaxation path. In the case of the isotopically diluted water surface, the bend (HOD)-stretching (OD) combination band couples with the free OH vibration, generating a relaxation rate similar to that in the pure water case. It is also found that the reorientation of the free OH bond contributes substantially to the relaxation of the free OH vibrational frequency component measured by TR-VSFG spectroscopy.

Possibility of realizing superionic ice VII in external electric fields of planetary bodies.

In a superionic (SI) ice phase, oxygen atoms remain crystallographically ordered while protons become fully diffusive as a result of intramolecular dissociation. Ice VII's importance as a potential candidate for a SI ice phase has been conjectured from anomalous proton diffusivity data. Theoretical studies indicate possible SI prevalence in large-planet mantles (e.g., Uranus and Neptune) and exoplanets. Here, we realize sustainable SI behavior in ice VII by means of externally applied electric fields, using state-of-the-art nonequilibrium ab initio molecular dynamics to witness at first hand the protons' fluid dance through a dipole-ordered ice VII lattice. We point out the possibility of SI ice VII on Venus, in its strong permanent electric field.

Nonequilibrium ab initio molecular-dynamics simulations of lattice thermal conductivity in irradiated glassy Ge2Sb2Te5

An analysis of thermal transients from nonequilibrium ab initio molecular-dynamics simulations can be used to calculate the thermal conductivity of materials with a short phonon mean-free path. We adapt the approach-to-equilibrium methodology to the three-dimensional case of a simulation that consists of a cubic core region at higher temperature approaching thermal equilibrium with a thermostatted boundary. This leads to estimates of the lattice thermal conductivity for the glassy state of the phase-change memory material, Ge2Sb2Te5, which are close to previously reported experimental measurements. Self-atom irradiation of the material, modeled using thermal spikes and stochastic-boundary conditions, results in glassy models with a significant reduction of diffusive thermal transport compared to the pristine glassy structure. This approach may prove to be useful in technological applications, e.g., for the suppression of thermal cross talk in phase-change memory and data-storage devices.

Insight of the thermal conductivity of ϵ-iron at Earth’s core conditions from the newly developed direct ab initio methodology

The electronic thermal conductivity of iron at the Earth’s core conditions is an extremely important physical property in the geophysics field. However, the exact value of electronic thermal conductivity of iron under extreme pressure and temperature still remains poorly known both experimentally and theoretically. A few recent experimental studies measured the value of the electronic thermal conductivity directly and some theoretical works have predicted the electronic thermal conductivity of iron at the Earth’s core conditions based on the Kubo-Greenwood method. However, these results differ largely with each other. A very recent research has confirmed that for iron at the Earth’s core conditions, the strength of electron-electron scattering could be comparable to that for electron-phonon scattering, meaning that the electron-electron scattering should also be considered when evaluating the electronic thermal conductivity in the Earth’s core situations. Here, by utilizing a newly developed methodology based on direct non-equilibrium ab initio molecular dynamics simulation coupled with the concept of electrostatic potential oscillation, we predict the electronic thermal conductivity of iron in h.c.p. phase. Our methodology inherently includes the electron-phonon and electron-electron interactions under extreme conditions. Our results are comparable to the previous theoretical and experimental studies. More importantly, our methodology provides a new physical picture to describe the heat transfer process in ϵ-iron at the Earth’s core conditions from the electrostatic potential oscillation point of view and offers a new approach to study the thermal transport property of pure metals in the planet’s cores with different temperature and pressure.

Communication: Influence of external static and alternating electric fields on water from long-time non-equilibrium ab initio molecular dynamics

The response of water to externally applied electric fields is of central relevance in the modern world, where many extraneous electric fields are ubiquitous. Historically, the application of external fields in non-equilibrium molecular dynamics has been restricted, by and large, to relatively inexpensive, more or less sophisticated, empirical models. Here, we report long-time non-equilibrium ab initio molecular dynamics in both static and oscillating (time-dependent) external electric fields, therefore opening up a new vista in rigorous studies of electric-field effects on dynamical systems with the full arsenal of electronic-structure methods. In so doing, we apply this to liquid water with state-of-the-art non-local treatment of dispersion, and we compute a range of field effects on structural and dynamical properties, such as diffusivities and hydrogen-bond kinetics.

Insight into the collective vibrational modes driving ultralow thermal conductivity of perovskite solar cells

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula $AM{X}_{3}$, where the $A$ cations occupy the hollows formed by the $M{X}_{3}$ octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3})$ with direct nonequilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+})$ and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. We also reveal the strongly localized phonons associated with the internal motions of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster, which contribute little to the total thermal conductivity. The importance of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

Methodology for determining the electronic thermal conductivity of metals via direct nonequilibriumab initiomolecular dynamics

Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential of the ion cores. Based on this premise, we develop a methodology for evaluating electronic thermal conductivity of metals by combining the free electron model and nonequilibrium ab initio molecular dynamics simulations. We confirm that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation, which is induced by the thermal motion of ion cores. This method directly predicts the electronic thermal conductivity of pure metals with a high degree of accuracy, without explicitly addressing any complicated scattering processes of free electrons. Our methodology offers a route to understand the physics of heat transfer by electrons at the atomistic level. The methodology can be further extended to the study of similar electron-involved problems in materials, such as electron-phonon coupling, which is underway currently.

Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibriumab initiomolecular dynamics

We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics (NE-AIMD), and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored-atom) is accelerated toward the neighboring defect-site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k_{NE} increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k_{NE}(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures, and reaches a factor of four orders of magnitude at the lowest temperature considered in the present study.

Lattice thermal conductivity of UO2 using ab-initio and classical molecular dynamics

We applied the non-equilibrium ab-initio molecular dynamics and predict the lattice thermal conductivity of the pristine uranium dioxide for up to 2000 K. We also use the equilibrium classical molecular dynamics and heat-current autocorrelation decay theory to decompose the lattice thermal conductivity into acoustic and optical components. The predicted optical phonon transport is temperature independent and small, while the acoustic component follows the Slack relation and is in good agreement with the limited single-crystal experimental results. Considering the phonon grain-boundary and pore scatterings, the effective lattice thermal conductivity is reduced, and we show it is in general agreement with the sintered-powder experimental results. The charge and photon thermal conductivities are also addressed, and we find small roles for electron, surface polaron, and photon in the defect-free structures and for temperatures below 1500 K.