Results Of Ab Initio Molecular Dynamics Research Articles

The structure of water-ammonia mixtures from classical and abinitio molecular dynamics.

The structure of aqueous ammonia solutions is investigated through classical molecular dynamics (MD) and abinitio molecular dynamics (AIMD) simulations. We have preliminarily compared three well-known classical force fields for liquid water (SPC, SPC/E, and TIP4P) in order to identify the most accurate one in reproducing AIMD results obtained at the Generalized Gradient Approximation (GGA) and meta-GGA levels of theory. Liquid ammonia has been simulated by implementing an optimized force field recently developed by Chettiyankandy et al. [Fluid Phase Equilib. 511, 112507 (2020)]. Analysis of the radial distribution functions for different ammonia concentrations reveals that the three water force fields provide comparable estimates of the mixture structure, with the SPC/E performing slightly better. Although a fairly good agreement between MD and AIMD is observed for conditions close to the equimolarity, at lower ammonia concentrations, important discrepancies arise, with classical force fields underestimating the number and strength of H-bonds between water molecules and between water and ammonia moieties. Here, we prove that these drawbacks are rooted in a poor sampling of the configurational space spanned by the hydrogen atoms lying in the H-bonds of H2O⋯H2O and, more critically, H2O⋯NH3 neighbors due to the lack of polarization and charge transfer terms. This way, non-polarizable classical force fields underestimate the proton affinity of the nitrogen atom of ammonia in aqueous solutions, which plays a key role under realistic dilute ammonia conditions. Our results witness the need for developing more suited polarizable models that are able to take into account these effects properly.

Promising AlN-modified VS2 heterostructure for Li-ion battery anode with high specific capacity

Degradation of the interior structure and the formation of uncontrolled Li dendrites at the negative electrode during the process of detaching and embedding Li atoms in lithium-ion batteries (LIBs) can negatively impact the cycling operation of the battery. To address these shortcomings, we constructed AlN/VS2 heterostructure with the expectation that it can be deployed as anodes in LIBs. Our results show that the AlN/VS2 heterostructure is characterized by strong stability, enhanced electrical conductivity and mechanical properties compared to monolayer AlN and VS2. Small Li clusters and cross-layer adsorption models are not thermodynamically prone to form, implying that the anode material plays a positive role in inhibiting the formation of dendrites. Moreover, the specific capacity of the AlN/VS2 heterostructure is 816.2 mAh/g, which is higher than that of monolayer VS2 (466 mAh/g) and graphene (372 mAh/g). The results of ab-initio molecular dynamics (AIMD) and biaxial stretching further demonstrate the structural and electrochemical stability of the electrode material during the embedding of Li atoms. To sum up, the AlN/VS2 heterostructure show potential as a high-performance anode for utilization in LIBs.

Lattice thermal conductivity and mechanical properties of the single-layer penta-NiN2 explored by a deep-learning interatomic potential.

Penta-NiN2, a novel pentagonal 2D sheet with potential nanoelectronic applications, is investigated in terms of its lattice thermal conductivity, stability, and mechanical behavior. A deep learning interatomic potential (DLP) is firstly generated from ab initio molecular dynamics (AIMD) data and then utilized for classical molecular dynamics simulations. The DLP's accuracy is verified, showing strong agreement with AIMD results. The dependence of thermal conductivity on size, temperature, and tensile strain, reveals important insights into the material's thermal properties. Additionally, the mechanical response of penta-NiN2 under uniaxial loading is examined, yielding a Young's modulus of approximately 368 GPa. The influence of vacancy defects on mechanical properties is analyzed, demonstrating a significant reduction in modulus, fracture stress, and ultimate strength. This study also investigates the influence of strain on phonon dispersion relations and phonon group velocity in penta-NiN2, shedding light on how alterations in the atomic lattice affect the phonon dynamics and, consequently, impact the thermal conductivity. This investigation showcases the ability of deep learning-based interatomic potentials in studying the properties of 2D penta-NiN2.

A combined AIMD and DFT study of the low-energy radiation responses of GaN.

Although GaN is a promising candidate for semiconductor devices, degradation of GaN-based device performance may occur when the device is bombarded by high-energy charged particles during its application in aerospace, astronomy, and nuclear-related areas. It is thus of great significance to explore the influence of irradiation on the microstructure and electronic properties of GaN and to reveal the internal relationship between the damage mechanisms and physical characteristics. Using a combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) study, we explored the low-energy recoil events in GaN and the effects of point defects on GaN. The threshold displacement energies (Eds) significantly depend on the recoil directions and the primary knock-on atoms. Moreover, the Ed values for nitrogen atoms are smaller than those for gallium atoms, indicating that the displacement of nitrogen dominates under electron irradiation and the created defects are mainly nitrogen vacancies and interstitials. The formation energy of nitrogen vacancies and interstitials is smaller than that for gallium vacancies and interstitials, which is consistent with the AIMD results. Although the created defects improve the elastic compliance of GaN, these radiation damage states deteriorate its ability to resist external compression. Meanwhile, these point defects lead the Debye temperature to decrease and thus increase the thermal expansion coefficients of GaN. As for the electronic properties of defective GaN, the point defects have various effects, i.e., VN (N vacancy), Gaint (Ga interstitial), Nint (N interstitial), and GaN (Ga occupying the N lattice site) defects induce the metallicity, and NGa (N occupying the Ga lattice site) defects decrease the band gap. The presented results provide underlying mechanisms for defect generation in GaN, and advance the fundamental understanding of the radiation resistances of semiconductor materials.

Water-air interface revisited by means of path-integral ab initio molecular dynamics.

Although nuclear quantum effects (NQEs) have been considered on bulk liquid water, the impact of these latter on the air-water interface has not yet been reported. Herein, by performing and comparing ab initio molecular dynamics (AIMD) and path integral AIMD (PI-AIMD) simulations, we reveal the impact of NQEs on structural, dynamical and electronic properties as well as IR spectra of the air-water interface at room temperature. NQEs, being able to describe a more accurate proton delocalization in H-bonded system than AIMD, reveal a different structural arrangement and dynamical behaviour of both bulk and interfacial water molecules in comparison to AIMD results. A more de-structured and de-bound water arrangement and coordination are identified when the quantum nature of nuclei are considered for both bulk and interfacial water molecules. Structural properties, such as inter-/intra-molecular bond lengths, coordination numbers and H-bonding angles of bulk and interfacial water molecules here calculated, are affected by NQEs mitigating the overstructured description given by AIMD. Further evidences of an AIMD overstructured description of bulk water are in the computed IR spectra, where an increased absorption peak intensity and an increased strength of the hydrogen-bond network are alleviated by NQEs. In addition, NQEs show a valuable impact on the electronic structure of the air-water interface, reducing the total valence bandwidth and the electronic energy band-gap when passing from bulk to interfacial water. This work proves how NQEs significantly affect properties and features of the air-water interface, that are essential to accurately describe H-bonded interfacial systems.

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.

Unveiling the dynamic thermal separation process of CO2 on the surface of calcium oxide: An ab-initio molecular dynamics study with experimental verification

Heating to release CO2 captured by Calcium oxide is one of the crucial steps in CO2 capture technology. However, the molecular mechanism underlying CO2 thermal desorption and the impact of CaO hydration on CO2 desorption remain unclear. Based on ab initio molecular dynamics (AIMD), a comprehensive analysis of the desorption and regeneration mechanism of carbonized CaO surfaces was conducted. Temperature is the primary factor responsible for CO2 thermal desorption on CaO surfaces. Simulations of CO2 thermal-dynamic desorption indicate that the desorption temperature range of CO2 from a dry CaO surface was between 400 and 500 °C. The explicit solvent model was utilised to simulate the desorption of CO2 under hydration condition. In the temperature range of 25–800 °C, the desorption behaviour of CO2 is substantially inhibited in the presence of water layers and Ca(OH)2. Consistent with the AIMD results, the thermal desorption experiment following CO2 adsorption confirmed that the desorption temperature of CO2 increases considerably when the adsorbed sample hydrates. Consequently, the desorption process of CO2 on CaO surface was comprehensively elucidated by a combination of theoretical study and experimental validation, thereby guiding the development and regeneration of CaO-based adsorbents.

Computational Investigations of the Water Structure at the α-Al2O3(0001)-Water Interface.

The α-Al2O3(0001)-water interface is investigated using ab initio molecular dynamics (AIMD) simulations. The spectral signatures of the vibrational sum frequency generation (vSFG) spectra of OH stretching mode for water molecules at the interface are related to the interfacial water orientation, hydrogen bond network, and water dissociation process at different water/alumina interfaces. Significant differences are found between alumina surfaces at different hydroxylation levels, namely, Al-terminated and O-terminated α-Al2O3(0001). By calculating the vibrational sum frequency generation spectrum and its imaginary component from AIMD results, the structure of interfacial waters as well as the termination of alumina slab are related to the spectral signatures of vSFG data.

Molecular dynamics study of fission product behaviour in molten [formula omitted]-[formula omitted] salt

The solvation of fission products in fuel salts and the effects due to their accumulation are important factors to understand in molten salt reactor design. In this contribution, we use ab initio Molecular Dynamics (AIMD) and polarizable ion model (PIM) based MD to study the structural and dynamic characteristics of the fission products (Cs, I and Xe) in the model fuel salt ThF4-LiF with different compositions at the infinite dilution limit. We first develop or refine existing PIM potentials to better reproduce the results of AIMD simulations. The AIMD and PIM-MD results are then compared in terms of the structures present in the salt, and the dynamics of their coordination. The results of this study indicate that, at 1200 K and 1400 K the fission products studied here have weak interactions with all composition ranges of fuel salt. This is evident, as first coordination shell associations being generally short-lived, as determined by cage correlation times. The diffusion coefficients determined through PIM-MD match the AIMD results reasonably well. The three fission products studied here are diffuse at similar rates; however, the Cs+ diffusion is slightly lower likely due to its more tightly bound associated F−. With all solutes, the diffusion coefficients decrease with a greater ThF4 fraction. Additionally, the behaviour of these solutes was studied at finite concentration ranges, up to 4 atom %. Here, it was found that the presence of Cs+ has little effect on the density, heat capacity and viscosity. However, these properties were affected by higher I+ and Xe concentrations.

Mechanical, Electronic, and Optical Properties of 8-16-4 Graphyne: A 2D Carbon Allotrope with Dirac Cones

Graphene’s success has led to the theoretical prediction and experimental synthesis of various 2D carbon-based allotropes. To explore the mechanical, structural, electronic, and optical properties of 8-16-4 graphyne, we employed density functional theory using the GGA/PBE approach, ab initio molecular dynamics (AIMD), and classical reactive molecular dynamics simulations. Our AIMD results indicate that this material demonstrates excellent dynamical and thermal stabilities. It has a formation energy of −8.57 eV/atom and an elastic moduli of 262.37 GPa. Regarding its band structure, this graphyne analogue is a semimetal with two Dirac cones. It also exhibits transparency and intense optical activity in the infrared region. Notably, the band structure of 8-16-4 graphyne remains practically unchanged at moderate strain regimes. To our knowledge, this is the first known 2D carbon allotrope to exhibit such behavior.

Ab Initio Derived Classical Force Field for Molecular Dynamics Simulations of ZnO Surfaces in Biological Environment.

Zinc oxide nanostructures are used in an ever increasing line of applications in technology and biomedical fields. This requires a detailed understanding of the phenomena that occur at the surface particularly in aqueous environments and in contact with biomolecules. In this work, we used ab initio molecular dynamics (AIMD) simulations to determine structural details of ZnO surfaces in water and to develop a general and transferable classical force field for hydrated ZnO surfaces. AIMD simulations show that water molecules dissociate near unmodified ZnO surfaces, forming hydroxyl groups at about 65% of the surface Zn atoms and protonating 3-coordinated surface oxygen atoms, while the rest of the surface Zn atoms bind molecularly adsorbed waters. Several force field atom types for ZnO surface atoms were identified by analysis of the specific connectivities of atoms. The analysis of the electron density was then used to determine partial charges and Lennard-Jones parameters for the identified force field atom types. The obtained force field was validated by comparison with AIMD results and with available experimental data on adsorption and immersion enthalpies, as well as adsorption free energies of several amino acids in methanol. The developed force field can be used for modeling of ZnO in aqueous and other fluid environments and in interaction with biomolecules.

Role of surface terminations in the chemical stability of CH3NH3PbI3 perovskite in combined light, H2O, and O2 environments: DFT/AIMD calculations and experimental validation

A deep understanding of the relationship between the surface termination of organic-inorganic metal halide perovskites and their structural degradation caused by environmental stimuli is critical for increasing the practicability of perovskite-based photovoltaic devices. In this work, the role of different surface termination on the degradation of CH3NH3PbI3 (MAPbI3) perovskite upon simultaneous exposure to water and oxygen in the presence of light is systematically investigated using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations methods. Our results show that photogenerated electrons increase the interaction of H2O and O2 molecules on the MAPbI3 perovskite and the coexistence of H2O and O2 molecules degrade the MAPbI3 perovskite surface faster than the presence of either monomer on the surface. AIMD results indicate that the PbI2-terminated MAPbI3 surface undergoes significant structural degradation upon simultaneous exposure to light, H2O, and O2, whereas the MAI-terminated surface is stable. Furthermore, a series of MAPbI3-based perovskite solar cells (PSCs) with different concentrations of MAI precursor are fabricated, and their photovoltaic performance is studied. Our experimental results show that the PSC treated with 3 mg mL−1 of MAI has achieved a maximum PCE of 16.16% and a larger VOC of 1.02 V, with correspondingly improved stability. The finding presented here provides a strategy for improving the quality of the MAPbI3 perovskite layer and its photovoltaic performance.

2D BN-biphenylene: structure stability and properties tenability from a DFT perspective.

With state-of-the-art density functional theory and ab initio molecular dynamics, we explored the BN-analog of a recently synthesized 2D biphenylene sheet. Its dynamical, thermal, and mechanical stability has been confirmed with phonon spectrum calculations, ab initio molecular dynamics (AIMD), and Born criteria, and its synthesis feasibility has been ascertained in terms of cohesive energy. The phonon spectrum and AIMD results show all positive frequencies and negligible variations in structure (bond length), kinetic and total energy for 5 ps, respectively. Having found this stable BN-BPh, we have analyzed its electronic, mechanical, optical, and vibration properties. Electronic property results (density of states and band structure) show that the BN-equivalent of metallic BPh has a large band gap of 3.12 eV and 4.48 eV with the PBE and HSE06 DFT functionals, respectively, similar to the graphene BN-equivalent (hBN) with a band gap of 6.1 eV. The mechanical strength of BN-BPh is close to that of BPh with a difference of ∼15 N m-1 and ∼10 N m-1 along the x- and y-directions, and its Young's modulus indicates that BN-BPh is stiffer than BN-graphyne. The optical absorption coefficient of BN-BPh shows a first peak at 4.48 eV (3.12 eV) with the HSE06 (PBE) functional. Infrared and Raman spectrum intensity and irreducible representation are computed, and the first IR (Raman) peak is found at 1300 cm-1 (1000 cm-1) for BN-BPh. Finally, substitutions of external atoms such as P, Al, As, C, Li, Mg, S, and Si for B and N atoms in BN-BPh tune its electronic properties to semiconducting, semi-metallic, and even metallic. Considering its mechanical, optical, and band gap tunability, BN-BPh might be useful in electronic, optical, and spintronics devices, even in high-pressure environments.

Ab initio canonical sampling based on variational inference

Finite temperature calculations, based on ab initio molecular dynamics (AIMD) simulations, are a powerful tool able to predict material properties that cannot be deduced from ground state calculations. However, the high computational cost of AIMD limits its applicability for large or complex systems. To circumvent this limitation we introduce a new method named Machine Learning Assisted Canonical Sampling (MLACS), which accelerates the sampling of the Born--Oppenheimer potential surface in the canonical ensemble. Based on a self-consistent variational procedure, the method iteratively trains a Machine Learning Interatomic Potential to generate configurations that approximate the canonical distribution of positions associated with the ab initio potential energy. By proving the reliability of the method on anharmonic systems, we show that the method is able to reproduce the results of AIMD with an ab initio accuracy at a fraction of its computational cost.

Viscosity in water from first-principles and deep-neural-network simulations

We report on an extensive study of the viscosity of liquid water at near-ambient conditions, performed within the Green-Kubo theory of linear response and equilibrium ab initio molecular dynamics (AIMD), based on density-functional theory (DFT). In order to cope with the long simulation times necessary to achieve an acceptable statistical accuracy, our ab initio approach is enhanced with deep-neural-network potentials (NNP). This approach is first validated against AIMD results, obtained by using the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional and paying careful attention to crucial, yet often overlooked, aspects of the statistical data analysis. Then, we train a second NNP to a dataset generated from the Strongly Constrained and Appropriately Normed (SCAN) functional. Once the error resulting from the imperfect prediction of the melting line is offset by referring the simulated temperature to the theoretical melting one, our SCAN predictions of the shear viscosity of water are in very good agreement with experiments.

Platinum equation of state to greater than two terapascals: Experimental data and analytical models

In order to develop a high impedance standard for shock physics applications, we investigated the equation of state (EOS) and shock compression of bulk platinum using canonical ab initio molecular dynamics (AIMD) simulations, coupled with experimental data on Sandia's Z machine. In simulations, we sampled the thermodynamic state space along isotherms ranging from 300 to 100 000 K for densities of 18 to 40 $\mathrm{g}/{\mathrm{cm}}^{3}$. While this rectilinear grid of EOS points was useful for preliminary experimental design, material going outside of the density/temperature space of the AIMD simulations required a SESAME-style, broad-range EOS, which we developed independently and subsequently compared to the AIMD simulations. In the final step we validated the SESAME EOS with experimental shock data to 2177 GPa on Sandia's Z machine. The theoretical and AIMD results are in excellent agreement with experiments.

Development of potentials for molecular dynamics simulations of dry and hydrated calcium aluminosilicate glasses by force matching and refinement

Potential development and refinement to improve the applicability and transferability of the forcefield is important to atomistic simulations of glass materials and their interactions with the environment. In this paper, we demonstrate potential development of calcium aluminosilicate glasses with and without water using a reactive potential framework by adopting the force matching technique and subsequent refinement. Accordingly, we have reproduced the forces on atoms of the system from a DFT based ab-initio molecular dynamics (AIMD) simulations by adjusting the potential parameters. We chose the diffuse charge reactive potential (DCRP) as the classical potential as it has been previously demonstrated to accurately and efficiently simulate interactions between water and silicate-glasses. The obtained force matched parameters were further refined to accurately reproduce the structures and dynamics of dry and hydrated calcium alumino silicate glass and gel structures. The Ca2+ behavior in solution was correctly reproduced with an average local coordination equal to 6.12 and a diffusion coefficient close to the experimental values. This force field also correctly reproduces the local environment of Al3+ in glass, but it falls short in describing Al3+ ion coordination in solutions as compared to AIMD results, suggesting a direction of future improvement. This set of potential can thus be used for simulations of calcium aluminosilicate glasses with various technological applications and hydrated gel structures resulted from corrosions of boroaluminosilicate glasses.

Transport properties of a quasisymmetric binary nitrogen-oxygen mixture in the warm dense regime.

Transport properties of mixtures in the warm dense matter (WDM) regime play an important role in natural astrophysics. However, a physical understanding of ionic transport properties in quasisymmetric liquid mixtures has remained elusive. Here, we present extensive ab initio molecular dynamics (AIMD) simulations on the ionic diffusion and viscosity of a quasisymmetric binary nitrogen-oxygen (N-O) mixture in a wide warm dense regime of 8-120 kK and 4.5-8.0 g/cm^{3}. Diffusion and viscosity of N-O mixtures with different compositions are obtained by using the Green-Kubo formula. Unlike asymmetric mixtures, the change of proportions in N-O mixtures slightly affects the viscosity and diffusion in the strong-coupling region. Furthermore, the AIMD results are used to build and verify a global pseudo-ion in jellium (PIJ) model for ionic transport calculations. The PIJ model succeeds in reproducing the transport properties of N-O mixtures where ionization has occurred, and provides a promising alternative approach to obtaining comparable results to AIMD simulations with relatively small computational costs. Our current results highlight the characteristic features of the quasisymmetric binary mixtures and demonstrate the applicability of the PIJ model in the WDM regime.

A new universal force-field for the Li2S-P2S5 system.

Lithium thiophosphate electrolyte is a promising material for application in all-solid-state batteries. Ab initio molecular dynamics (AIMD) simulations have been used to investigate the ion conduction mechanisms in single-crystalline and glassy compounds. However, the complexity of real materials (e.g., materials with grain boundaries and multiphase glass-ceramics) causes AIMD simulations to have high computational cost. To overcome this computational limitation, we developed a new interatomic potential for classical molecular dynamics (CMD) simulations of Li solid-state electrolytes. The training datasets were generated from representative sulfide electrolytes (β-Li3PS4, γ-Li3PS4, Li4P2S6, Li7P3S11, and Li7PS6 crystals and 70Li2S-30P2S5 glass). Using the functional forms of the Class II and Stillinger-Weber potentials, all parameters were optimized by minimizing the differences in forces on atoms, stresses, and potential energies between the CMD and AIMD results. Subsequent validation showed that the optimized parameters can reproduce the dynamics of Li+ as well as the structures of the crystalline and glassy materials. The ionic conductivity of Li7P3S11 crystal was approximately five times that of the isostoichiometric 70Li2S-30P2S5 glass, indicating that CMD simulations using the developed force-field accurately reproduced the effective conduction path in Li7P3S11 from AIMD. The developed force-field parameters make it possible to simulate complex materials including amorphous-crystalline interfaces and multiphase glass-ceramics in the CMD framework.

Ab initio and experimental study on the mechanism of Al4C3 nitridation in vacuum to prepare AlN

Aluminium nitride has good electrical, thermal, and mechanical properties, and Al4C3 is the most important intermediate in the preparation of AlN through carbothermal reduction. Ab initio molecular dynamics (AIMD) simulations were adopted in this study to elucidate the nitridation reaction mechanism of Al4C3, and an experimental study was conducted to verify the results of AIMD. Both the AIMD and experimental results showed that the nitridation pathway of Al4C3 was Al4C3→Al5C3N→AlN and that the nitridation of Al5C3N was considerably rapid. During the nitridation of Al4C3, the addition of carbon not only had a catalytic effect on the activation of nitrogen molecules but also promoted the aggregation of carbon atoms in the Al4C3 lattice to form elemental carbon.