Ab Initio Molecular Dynamics Method Research Articles

Concentration effects on dynamic fluctuations in structure and thermodynamic properties of LiCl–AlCl3 molten Salt: Insights from ab initio molecular dynamics

The short–range and mid–range structural features, as well as thermodynamic properties of a LiCl–AlCl3 mixed molten salt at 873 K, were investigated as a function of AlCl3 concentration using ab initio molecular dynamics methods. In all solutions, the short-range structure of Al3+ reveals a regular tetrahedral [AlCl4]– unit. These tetrahedral units exist in isolation within dilute AlCl3 solution and polymerize into dimers or trimers through Al–Cl–Al bridge bonds in concentrated solutions. The dynamic dissociation analysis of Al3+ and Li+ structures indicates that the Al–Cl bond considerably stronger than Li–Cl bond. The polymerization of the [AlCl4]– units leads to the dissociation of the Li+ coordination shells into freely isolated cations. Furthermore, we examined changes in density, self-diffusion coefficient, and ionic conductivity with varying AlCl3 concentration in terms of thermodynamic properties. The conductivity and self-diffusion coefficient of Li+ showed a noticeable peak at 50 % concentration. This peak may be associated with the aggregation of [AlCl4]– units and the dissociation of Li+ coordination shells.

The properties of radiation defects and threshold energies of displacement in zirconium hydride obtained by a new deep-learning potential

Abstract Zirconium hydride (ZrH2) is an ideal neutron moderator material. However, radiation effects significantly alter its properties, subsequently impacting its behavior and the lifespan of the reactor. The threshold energy of displacement is an important quantity of the number of radiation defects produced, which helps us to predict the evolution of radiation defects in ZrH2. Molecular dynamics (MD) and ab initio molecular dynamics (AIMD) are two main methods for calculating the threshold energy of displacement. MD simulations with empirical potentials often fail to accurately depict the transitional states that lattice atoms must surpass to reach an interstitial state. Additionally, the AIMD method does not afford large-scale calculation, posing a computational challenge beyond the scope of density functional theory simulations. Machine learning potentials are renowned for their high accuracy and efficiency, making them an increasingly preferred choice for molecular dynamics simulations. In this work, we developed an accurate potential energy model for the ZrH2 system by using the deep-potential (DP) method. The DP model has a high degree of agreement with first-principles calculations for the typic defect energies and mechanical properties of the ZrH2 system, including the basic bulk properties, formation energy of point defects, as well as diffusion behavior of hydrogen and zirconium. By integrating the DP model with Ziegler-Biersack-Littmark (ZBL) potential, we have predicted the threshold energies of displacement of zirconium and hydrogen in ε-ZrH2.

Ferroelectric Phase Transition in Barium Titanate Revisited with Ab Initio Molecular Dynamics.

The ferroelectric phase transition of the perovskite barium titanate as well as its technical importance regarding the switching of respective polar properties is well known and has been thoroughly studied, both experimentally and on theoretical grounds. While details about the phase diagram as well as transition temperatures are experimentally well known, the theoretical approaches still face difficulties in contributing a detailed description of these phase transitions. Within this work, a new methodological approach is introduced to revisit the ferroelectric phase transition with first-principles methods. With the chosen ab initio molecular dynamics (AIMD) method in combination with the applied NpT ensemble, we are able to join the accuracy of density functional theory (DFT) with ambient conditions, realized using a thermostat and barostat in an MD simulation. The derived phase diagram confirms recent corrections in the theoretical models and reproduces the phase boundary pressure dependence of TC. In conclusion of the statistical atomistic dynamics, the nature of the transition can be described in a more detailed way. In addition, this work paves the way towards locally patterned piezoelectrica by means of acoustic standing waves as well as piezoelectrically induced acoustic resonators.

Study on the Microscopic Mechanism of the Grain Refinement of Al-Ti-B Master Alloy

In the present work, the structure and properties of TinBn (n = 2–12) clusters were studied, and the microstructure of a Al-Ti-B system was simulated by molecular dynamics to determine the grain refinement mechanism of an Al-Ti-B master alloy in Al alloy. Based on the density functional theory method, the structural optimization and property calculations of TinBn (n = 2–12) clusters were carried out. The clusters at the lowest energy levels indicated that the Ti and B atoms were prone to form TiB2 structures, and the TiB2 structures tended to be on the surface of the clusters. The Ti10B10 cluster was determined to be the most stable structure in the range of n from 2 to 12 by average binding energy and second-order difference energy. The analysis of HOMOs and LUMOs suggested that TiB2 was the active center in the cluster; the activity of Ti was high, but the activity of B atoms decreased as the cluster size n increased. Meanwhile, the prediction of reaction sites by Fukui function, condensed Fukui function, and condensed dual descriptor identify that Ti atoms were more active than B atoms. Furthermore, TiB2 structures were found in the Al-Ti-B system simulated by the ab initio molecular dynamics method, and there were Al atoms growing on the Ti atoms in the TiB2. Based on the above analysis, this study suggests that TiB2 may be a heterogeneous nucleation center of α-Al. This work helps to further understand the mechanism of Al-Ti-B induced heterogeneous nucleation in Al alloys, which can provide theoretical guidance for related experiments.

Mechanism of ionic dissociation of HCl in the smallest water clusters.

The dissociation of strong acids into water is a fundamental process in chemistry and biology. Determining the minimum number of water molecules that can result in an ionic dissociation of hydrochloric acid (HCl → H+ + Cl-) remains a challenging subject. In this study, the reactions of H2O with HCl(H2O)n-1 (HCl-H2O cluster), i.e., HCl(H2O)n-1 + H2O (n = 3-7), were investigated by using the direct ab initio molecular dynamics (AIMD) method. Direct AIMD calculations were performed to set the collision energy of H2O to zero for all trajectories. For n = 3, no reaction occurred. In contrast, HCl dissociated to H+ + Cl- at n = 4, forming a contact ion pair (cIP) and solvent-separated ion pair (ssIP) as products. The reactions were expressed as HCl(H2O)3 + H2O → H3O+(H2O)2Cl- (ssIP), and HCl(H2O)3 + H2O → H3O+(Cl-)(H2O)2 (cIP). The ion pair (IP) products were dependent on the collision site of H2O relative to HCl(H2O)3. For n = 5-7, both IPs were formed through the reaction between H2O and HCl(H2O)n-1 (n = 5-7). The reaction between HCl and (H2O)4 (HCl + (H2O)4 → HCl(H2O)4) was non-reactive in IP formation. The reaction mechanism was discussed based on the theoretical results.

Ab Initio Molecular Dynamics Study on Self-Healing Solid Polymer Electrolyte for Lithium Metal Batteries

Lithium metal batteries (LMBs) have generally become potential candidates for energy storage devices because of their high energy density. However, the practical applications of LMBs have been limited due to the low safety of liquid electrolytes. Compared to liquid electrolytes, solid-state polymer electrolytes (SPEs) have excellent mechanical strength. However, there is room for improvement because traditional PEO-based SPEs need more mechanical properties and electrode contacts for flexible energy device applications. Self-healing solid polymer electrolytes (SHSPEs) have excellent flexibility and healing ability and can also suppress dendrite formation to increase the safety of LMBs. In this study, a novel SHSPE is designed by a combination of the zwitterionic copolymer, SBMA ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide)-co-BA (Butyl Acrylate), and a lithium salt, LiTFSI. We explored their solvation structures and reactivity on the Li metal anode using Density functional theory (DFT) and ab initio molecular dynamics (AIMD) methods. Besides, we also studied the reactivity of the considered SHSPE on the Cu (100) surface to explore the SEI formation mechanisms. The cationic NR4 + and anionic SO3 - groups of SBMA interact electrostatically and form cross-linked network structures within the electrolyte framework, providing the designed SHSPE with prominent self-healing capacity. The decomposition reactions of SHSPE on the Li metal anode are explored, and the resultant decomposition products that contribute to the formation of the solid electrolyte interphase (SEI) are identified. Furthermore, AIMD results indicate that highly concentrated SO3 - groups in the SHSPE promote Li ion diffusion, which increases Li ionic conductivity. The present findings provide design criteria for developing highly conductive self-healing SPEs.

Ab Initio Molecular Dynamics Study of Electron Excitation Effects on UO2 and U3Si.

In this study, an ab initio molecular dynamics method is employed to investigate how the microstructures of UO2 and U3Si evolve under electron excitation. It is found that the U3Si is more resistant to electron excitation than UO2 at room temperature. UO2 undergoes a crystalline-to-amorphous structural transition with an electronic excitation concentration of 3.6%, whereas U3Si maintains a crystalline structure until an electronic excitation concentration reaches up to 6%. Such discrepancy is mainly due to their different electronic structures. For insulator UO2, once valence U 5f electrons receive enough energy, they are excited to the conduction bands, which induces charge redistribution. Anion disordering is then driven by cation disordering, eventually resulting in structural amorphization. As for metallic U3Si, the U 5f electrons are relatively more difficult to excite, and the electron excitation leads to cation disordering, which eventually drives the crystalline-to-amorphous phase transition. This study reveals that U3Si is more resistant to electron excitation than UO2 under an irradiation environment, which may advance the understanding of related experimental and theoretical investigations to design radiation-resistant nuclear fuel uranium materials.

Machine learning force fields for molecular liquids: Ethylene Carbonate/Ethyl Methyl Carbonate binary solvent

Highly accurate ab initio molecular dynamics (MD) methods are the gold standard for studying molecular mechanisms in the condensed phase, however, they are too expensive to capture many key properties that converge slowly with respect to simulation length and time scales. Machine learning (ML) approaches which reach the accuracy of ab initio simulation, and which are, at the same time, sufficiently affordable hold the key to bridging this gap. In this work we present a robust ML potential for the EC:EMC binary solvent, a key component of liquid electrolytes in rechargeable Li-ion batteries. We identify the necessary ingredients needed to successfully model this liquid mixture of organic molecules. In particular, we address the challenge posed by the separation of scale between intra- and inter-molecular interactions, which is a general issue in all condensed phase molecular systems.

Empirically Optimized One-Electron Pseudopotential for the Hydrated Electron: A Proof-of-Concept Study.

Mixed quantum-classical molecular dynamics simulations have been important tools for studying the hydrated electron. They generally use a one-electron pseudopotential to describe the interactions of an electron with the water molecules. This approximation shows both the strength and weakness of the approach. On the one hand, it enables extensive statistical sampling and large system sizes that are not possible with more accurate ab initio molecular dynamics methods. On the other hand, there has (justifiably) been much debate about the ability of pseudopotentials to accurately and quantitatively describe the hydrated electron properties. These pseudopotentials have largely been derived by fitting them to ab initio calculations of an electron interacting with a single water molecule. In this paper, we present a proof-of-concept demonstration of an alternative approach in which the pseudopotential parameters are determined by optimizing them to reproduce key experimental properties. Specifically, we develop a new pseudopotential, using the existing TBOpt model as a starting point, which correctly describes the hydrated electron vertical detachment energy and radius of gyration. In addition to these properties, this empirically optimized model displays a significantly modified solvation structure, which improves, for example, the prediction of the partial molar volume.

Ab initio molecular dynamics of insulating paper: Mechanism of insulating paper cellobiose cracking at transient high temperature

Abstract Oil-impregnated paper is the most widely used insulating material for power transformers. Power transformers inevitably generate a lot of heat during operation. Among them, thermal aging is one of the main forms of aging for insulating paper. In this study, the ab initio molecular dynamics method based on the density functional theory is used to simulate the cracking mechanism of cellobiose under transient high temperature. The results show that the cellobiose is relatively stable at 343 K, the motility of the cellobiose is enhanced at 1,800 K, the cellobiose starts to decompose at 2,400 K, and new characteristic products are formed at 3,000 K. The characteristic products include CO, H2O, CH3OH, H2, and CH4. These characteristic products can represent the degree of cracking of insulating paper. Therefore, it is necessary to explore the mechanism of cracking of insulating paper caused by transient high temperature.

Lithium nitrate mediated dynamic formation of solid electrolyte interphase revealed by in situ Fourier transform infrared spectroscopy

A stable solid electrolyte interphase (SEI) plays a vital role in the cyclic stability and Coulombic efficiency (CE) of high-performance lithium-sulfur (Li-S) batteries. It is recognized that the LiNO3 additive can stabilize the SEI of the lithium electrode. However, the exact mechanism of the LiNO3 additive on the SEI of the lithium electrode remains unclear. In this work, we first revealed the mediation mechanism of LiNO3 additive on the dynamic evolution of the SEI on a lithium anode surface through in situ Fourier transform infrared (FTIR) spectroscopy and ab initio molecular dynamics (AIMD) methods. The FTIR and AIMD results directly proved that LiNO3 can accelerate the reduction of lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) into small molecules rich in Li2O on lithium anode, thus forming a compact and stable SEI after immersing in the LiNO3-containing electrolyte. Moreover, the decomposition of LiTFSI and the solvent is hindered in the subsequent lithium deposition stripping process due to the stable SEI, thus leading to higher Coulombic efficiency and long-term cyclic stability. In addition, an ROSO2Li-like intermediate is also observed during the lithium deposition process while decomposing or diffusing away during the lithium stripping process, maintaining a dynamic formation/dissolution equilibrium of the SEI. This research provides a new insight into understanding the role of LiNO3 in stabilizing lithium electrode.

The influence of site preference on the elastic properties of FCC_CoCrFeNi multi-principal element alloy

The influence of the existing site preference of the constituent elements on the sublattice on elastic properties is rarely explored in multi-principal element alloys (MPEAs). In this work, in order to explore the influence of the site preference of constituent atoms on the thermodynamic properties and elastic properties, the ordered configurations based on the previously predicted site occupying fractions (SOFs) and the disordered configuration based on a special quasi-random structure (SQS) were established, and the thermodynamic properties and elastic properties were predicted using the quasi-harmonic approximation (QHA) method first. It was found that the site preferring behaviors of atoms on the sublattices will not only improve the stability of the structure but will also lead to bigger elastic properties than its ideal disordered structure. Although the prediction of the temperature-dependent elastic properties using the QHA method shortens the time costs by only considering the effects of thermal expansion, it ignores the significant effect of lattice vibration on elastic properties at high temperatures. In order to explore the influence of lattice vibration on the temperature-dependent elastic properties further, the elastic properties of the ordered FCC_CoCrFeNi MPEA were predicted using the ab initio molecular dynamics (AIMD) method at several selective temperatures. The results show that except for the elastic constant C12 and bulk modulus, the lattice vibration reduces other elastic properties on the basis of thermal expansion. The predicted temperature-dependent shear and Young's moduli agree well with the limited experimental literature data. Thus, the temperature-dependent elastic properties can be reasonably predicted using the AIMD method based on site preference. The predicted polycrystalline elastic moduli B, G, and E of FCC_CoCrFeNi MPEA at 973 K are 146.68, 53.79, and 143.78 GPa, respectively.

Comparative analysis of molecular interactions in quaternary fluid system performed by classical and ab initio molecular dynamics

Molecular interactions in the quaternary fluid system acetic acid–n-propanol–n-propyl acetate–water were analyzed by classical and ab initio molecular dynamics methods. It was shown that ab initio molecular dynamics simulation can reproduce the molecular mobility tendency and structural features of a multicomponent system without empirical parameters.

Segregation of Alkaline Earth Atoms Affects Prenucleation at L-Al/γ-Alumina Interfaces

Segregation of foreign atoms on a solid substrate in a liquid metal modifies the geometry and chemistry of the substrate surface and, correspondingly, its potency to nucleate a solid metal. We here investigate the effects of the segregation of alkaline earth (AE) atoms, Mg, Sr, and Ba, at the interfaces between liquid Al and γ-Al2O3{1 1 1} substrates using an ab initio molecular dynamics method. This study reveals the high stability and localized nature of the segregated AE atoms at the oxide substrates. The segregation of the AE atoms induces reconstruction of the metal atoms terminating the oxide substrates, and causes atomic roughness of the substrate surfaces. The content of the induced atomic roughness relates to the ionic size of the AE atoms. Correspondingly, the potency of the oxide substrates is modified. This indicates the possibility of manipulating the substrate potency via segregation of selected impure atoms, which would help to control solidification processes.

Inhibition Mechanism of MgO Addition on High‐Temperature Oxidation of Magnetite: Density Functional Theory and Ab Initio Molecular Dynamics Methods Joint Research and Experimental Verification

In order to investigate and analyze the effect of MgO gangue on the surface adsorption and oxidation behavior of magnetite, the physicochemical properties of substances in the magnetite oxidation process are investigated in this study by using thermogravimetric experiments, density functional theory, and ab initio molecular dynamics methods (AIMD) methods. The oxidation mechanism of magnetite and the inhibition mechanism of magnesium oxide on the oxidation properties of magnetite are elucidated. The results of the thermogravimetric experimental study show that the initial oxidation temperature of magnetite tends to increase with the increase of MgO content. At the same time, the presence of MgO leads to the migration of oxidation reactions to the high‐temperature region. AIMD study shows that the presence of the gangue element Mg prolongs the adsorption and dissociation time of O2 molecules on the surface and reduces the interfacial chemical reaction rate of Fe3O4. Moreover, the chemical bonds formed in the system after doping Mg atom are more stable, which is not conducive to the migration of oxygen atoms between Fe3O4 crystal structures, thus affecting the comprehensive oxidation performance of minerals.

Ab initio molecular dynamics calculation and vacuum distillation experimental study on the interaction mechanism of PbS–FeS

An important way to obtain PbS and FeS is the thermal decomposition of Jamesonite. However, the mechanism of PbS–FeS interaction is unknown. In order to explore the separation effect of PbS and FeS, thermodynamic calculation, dynamic simulation and vacuum distillation experiments were used to explore the interaction mechanism of PbS and FeS. The decomposition and volatilization temperatures of PbS and FeS were obtained by thermodynamic calculation. The difference of saturated vapor pressure indicates that there is a tendency of separation between them. The ab initio molecular dynamics method was used to calculate the density of states of PbS and FeS at different temperatures, indicating that the FeS(114) structure is more stable than the PbS(200) structure, and the Pb–S bond is easier to break than the Fe–S bond at the same temperature. The experimental results of vacuum distillation show that the volatilization temperature of PbS is much lower than that of FeS, and FeS can only volatilize into the gas phase at a higher temperature. The theoretical calculation method is consistent with the experimental results, which shows that the thermodynamic calculation and dynamic simulation methods are reliable in predicting the volatilization and separation of crystal structures. The interaction mechanism between PbS and FeS was explored from the theoretical point of view to provide theoretical guidance for vacuum distillation experiment.

Theoretical insights into the coordination structures, stabilities and electronic spectra of Cm3+ species at the gibbsite-water interface: A computational study combing ab initio molecular dynamics and wave function theory

The information of structure and stability of actinide species is key to understand the sorption mechanism of actinides at mineral–water interface. Such information is approximately derived from experimental spectroscopic measurements and needs to be accurately obtained by a direct atomic-scale modelling. Herein, systematic first-principles calculations and ab initio molecular dynamics (AIMD) simulations are carried out to study the coordination structures and absorption energies of Cm(III) surface complexes at gibbsite-water interface. Eleven representative complexing sites are investigated. The most stable Cm3+ sorption species are predicted to be a tridentate surface complex in weakly acidic/neutral solution condition and a bidentate one in the alkaline solution condition. Moreover, luminescence spectra of the Cm3+ aqua ion and the two surface complexes are predicted based on the high-accuracy ab initio wave function theory (WFT). The results give a gradually decreasing emission energy in good agreement with experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. This work is a comprehensive computational study involving AIMD and ab initio WFT methods to gain the coordination structures, stabilities, and electronic spectra of actinide sorption species at the mineral–water interface, thus providing important theoretical support for geological disposal of actinide waste.

The hydration of Li+ and Mg2+ in subnano carbon nanotubes using a multiscale theoretical approach.

The separation of brines with high Mg/Li mass ratios is a huge challenge. To provide a theoretical basis for the design of separation materials, the hydration of Li+ and Mg2+ in confinement using carbon nanotubes (CNTs) as the 1-D nanopore model was investigated using a multiscale theoretical approach. According to the analysis of the first coordination layer of cations, we determined that the coordination shells of two cations exist inside CNTs, while the second coordination shells of the cations are unstable. Moreover, the results of the structure analysis indicate that the hydration layer of Li+ is not complete in CNTs with diameters of 0.73, 0.87, and 1.00nm. However, this does not occur in the 0.60nm CNT, which is explained by the formation of contact ion pairs (CIP) between Li+ and Cl- that go through a unstable solvent-shared ion pair [Li(H2O)4]+, and this research was further extended by 400ns in the 0.60nm CNT to address the aforementioned results. However, the hydration layer of Mg2+ is complete and not sensitive to the diameter of CNTs using molecular dynamics simulation and an ab initio molecular dynamics (AIMD) method. Furthermore, the results of the orientation distribution of Li+ and Mg2+ indicate that the water molecules around Mg2+ are more ordered than water molecules around Li+ in the CNTs and are more analogous to the bulk solution. We conclude that it is energetically unfavorable to confine Li+ inside the 0.60-nm diameter CNT, while it is favorable for confining Li+ inside the other four CNTs and Mg2+ in all CNTs, which is driven by the strong electrostatic interaction between cations and Cl-. In addition, the interaction between cations and water molecules in the five CNTs was also analyzed from the non-covalent interaction (NCI) perspective by AIMD.

Local Structure of Molten Lithium Fluoride. II. Structure Factor Calculation by Ab Initio and Classical Molecular Dynamics Methods

Local Structure of Molten Lithium Fluoride. II. Structure Factor Calculation by Ab Initio and Classical Molecular Dynamics Methods

Oxidation resistance of TiSiN and AlCrN hard-coatings: Ab initio calculations and experiments

Understanding the oxidation process of hard-coating at the atomic scale is still limited. In this work, the high temperature oxidation behaviors of Ti0.86Si0.14N and Al0.66Cr0.34N coatings have been studied by ab initio molecular dynamics (AIMD) and experimental methods. The average displacement height of metal atoms bonded to O is used to reveal the morphology of oxide layers on the surface. The displacement of bonded metal atoms in ferromagnetic states changed and gradually closed to overlap, demonstrating that it is necessary to consider the magnetic property of Cr in the calculation of AlCrN system. In order to verify the simulation results, the physical vapor deposition (PVD) was used to prepare Ti0.86Si0.14N and Al0.66Cr0.34N hard coatings, and the microstructures of oxide layer at corresponding annealing temperatures are studied. It was found that the oxide layer of Ti0.86Si0.14N coatings gradually changes from a monolayer to a double layer with Ti-rich layer in the outmost as temperature increases. And the oxidation process of Al0.66Cr0.34N coating is controlled by the competition mechanism of Al-O bond and Cr-O bond. The experimental results reproduced the simulatied ones, verifying the feasibility of ab initio molecular dynamics method in quicker predicting the oxidation resistance of coatings.