Results Of Ab Initio Molecular Dynamics Research Articles

Ab initio and force field molecular dynamics study of bulk organophosphorus and organochlorine liquid structures.

We performed ab initio molecular dynamics (AIMD) simulations to benchmark bulk liquid structures and to evaluate results from all-atom force field molecular dynamics (FFMD) simulations with the generalized Amber force field (GAFF) for organophosphorus (OP) and organochlorine (OC) compounds. Our work also addresses the current and important topic of force field validation, applied here to a set of nonaqueous organic liquids. Our approach differs from standard treatments, which validate force fields based on thermodynamic data. Utilizing radial distribution functions (RDFs), our results show that GAFF reproduces the AIMD-predicted asymmetric liquid structures moderately well for OP compounds that contain bulky alkyl groups. Among the OCs, RDFs obtained from FFMD overlap well with AIMD results, with some offsets in position and peak structuring. However, re-parameterization of GAFF for some OCs is needed to reproduce fully the liquid structures predicted by AIMD. The offsets between AIMD and FFMD peak positions suggest inconsistencies in the developed force fields, but, in general, GAFF is able to capture short-ranged and long-ranged interactions of OPs and OCs observed in AIMD. Along with the local coordination structure, we also compared enthalpies of vaporization. Overall, calculated bulk properties from FFMD compared reasonably well with experimental values, suggesting that small improvements within the FF should focus on parameters that adjust the bulk liquid structures of these compounds.

First-Principles Study of Layered Transition Metal Oxides As a Cathode Material for Ca-Ion Intercalating Batteries

The Ca-based rechargeable batteries are a promising direction in multivalent energy storage that requires development of the intercalating cathode materials capable of high voltage and capacity. Given that monovalent alkaline metals (e.g. K, Na) with similar ionic radius of Ca can be reversely stored in layered transition metal (TM) oxides, we infer that divalent Ca-ion can be as well. Here, we demonstrate on the basis of first-principles calculations that layered TM oxides exhibit favorable thermodynamic and kinetic properties that enable topotactic Ca intercalating reactions. From the calculated phase diagram, the layered CaXCo2O4with space groups of P1 and P21/m are stable at multiple Ca concentrations x ranging from 0 to 1 and show a smooth voltage plateau higher than 3V up to x=0.5. The Ca migration barriers calculated by nudged elastic band method can be as low as 0.36 eV and 0.27 eV at the dilute and high vacancy concentration limits, respectively. At the intermediate vacancy concentration, the calculated migration barriers depend on the local vacancy environments of diffusing atom, and stochastic analysis of Ca hopping events performed by ab initio molecular dynamics (AIMD) simulation has shown that the migration barriers are still lower than 0.32 eV. Therefore, the Ca diffusivity at room temperature extrapolated from the AIMD results is comparable to Li diffusivity, suggesting feasibility of use of layered CaxCo2O4 as multivalent cathode materials. We will also discuss the trends in thermodynamic and kinetic properties as the TM in the layered structure is substituted with other TM species.

Thermodynamic and transport properties of meteor melt constituents from ab initio simulations: MgSiO3, SiO2, and MgO

Recent experiments have suggested that melt flow plays a critical role in the ablation of meteoroids during atmospheric entry. Thus, modeling ablation requires knowledge of the melt properties of meteoritic constituents. These properties, however, are poorly understood and difficult to obtain with experimental techniques at entry conditions. An alternative means of obtaining high-temperature melt properties is through ab initio molecular dynamics (AIMD) simulations. Such simulations are performed here to characterize the melt properties of enstatite (MgSiO3), which is prevalent in certain types of chondrites, and its constitutive oxides (SiO2 and MgO). The structure, thermodynamic properties (density, bulk modulus, heat capacity, and coefficient of thermal expansion), and transport properties (diffusion and viscosity) are computed across the entire liquid phase and agree well with the limited number of available experiments. The high fidelity AIMD results are compared against less accurate models for melt property determination, which include classical molecular dynamics simulations and empirical mixture rules. Properties obtained from these alternative models generally show large relative errors compared to experiment, with viscosity, in particular, having errors of up to 98%. The present results highlight the potential of AIMD simulations to provide “quantitatively accurate” properties for melts of complex silicates found in meteorites and terrestrial rocks.

A comparison of the thermal decomposition mechanism of wurtzite AlN and zinc blende AlN

The sublimation route is one of the primary and most significant methods for the synthesis of an aluminum nitride (AlN) single crystal. Its long synthesis time and high reaction temperature, however, limit the production of its commercial product. In this work, we applied HSC Chemistry 6 software, ab initio molecular dynamics, and X-ray diffraction to investigate the thermal decomposition of AlN. We calculated the decomposition temperatures of AlN under vacuum and simulated the decomposition mechanism of AlN by the ab initio molecular dynamics method. According to the thermodynamic calculations, the decomposition temperature of AlN decreased following a decrease in the system pressure. The ab initio molecular dynamics results indicated that wurtzite-type AlN (w-AlN) was decomposed by the layer-by-layer mechanism and followed a decomposition reaction equation of AlN → Al(g) + 0.29N2(g) + 0.42N(g), which originated from the inequality sp3 hybridization. The zinc-type AlN (z-AlN) decomposed from the surface to interior of the structure because of the equality of the sp3 hybridization, and the z-AlN decomposition reaction equation followed AlN → Al(g) + 0.5N2(g). The AlN decomposition experiments further verified that Al(g) was the product of the wurtzite-type AlN thermal decomposition. This work can provide valuable information for the preparation of the AlN single crystal.

Shock compression of strongly correlated oxides: A liquid-regime equation of state for cerium(IV) oxide

The shock Hugoniot for full-density and porous ${\mathrm{CeO}}_{2}$ was investigated in the liquid regime using ab initio molecular dynamics (AIMD) simulations with Erpenbeck's approach based on the Rankine-Hugoniot jump conditions. The phase space was sampled by carrying out NVT simulations for isotherms between 6000 and 100 000 K and densities ranging from $\ensuremath{\rho}=2.5$ to $20\phantom{\rule{0.28em}{0ex}}\mathrm{g}/{\mathrm{cm}}^{3}$. The impact of on-site Coulomb interaction corrections $+U$ on the equation of state (EOS) obtained from AIMD simulations was assessed by direct comparison with results from standard density functional theory simulations. Classical molecular dynamics (CMD) simulations were also performed to model atomic-scale shock compression of larger porous ${\mathrm{CeO}}_{2}$ models. Results from AIMD and CMD compression simulations compare favorably with Z-machine shock data to 525 GPa and gas-gun data to 109 GPa for porous ${\mathrm{CeO}}_{2}$ samples. Using results from AIMD simulations, an accurate liquid-regime Mie-Gr\"uneisen EOS was built for ${\mathrm{CeO}}_{2}$. In addition, a revised multiphase SESAME-type EOS was constrained using AIMD results and experimental data generated in this work. This study demonstrates the necessity of acquiring data in the porous regime to increase the reliability of existing analytical EOS models.

Evolution of the atomic structure of a supercooled Zr55Cu35Al10 liquid

Structural data from the supercooled Zr55Cu35Al10 ternary alloy liquid were collected in-situ in the temperature range from 1172 K to 953 K during continuous cooling using the high-energy synchrotron X-ray Advanced Photon Source. The lowest-r peaks of the pair distribution functions (PDFs) sharpened, becoming higher and narrower, with decreasing temperature. The experimental data are in good agreement with the results of ab-initio molecular dynamics (AIMD) simulations. Quantitative analyses of the simulated structures (clusters, interconnecting zones and free volume) in the supercooled liquid were made based on the tight-bond cluster model. The significance of Al additions on the increase in glass forming ability (GFA) was estimated from the simulated structures and their electronic structure analysis.

Impact of oxygen on the 300-K isotherm of Laser Megajoule ablator using ab initio simulation.

The ablator material for inertial confinement fusion (ICF) capsules on the Laser Mégajoule is a glow-discharge polymer (GDP) plastic. Its equation of state (EOS) is of primary importance for the design of such capsules, since it has direct consequences on shock timing and is essential to mitigate hydrodynamic instabilities. Using ab initio molecular dynamics (AIMD), we have investigated the 300-K isotherm of amorphous CH(1.37)O(0.08) plastic, whose structure is close to GDP plastic. The 300-K isotherm, which is often used as a cold curve within tabular EOS, is an important contribution of the EOS in the multimegabar pressure range. AIMD results are compared to analytic models within tabular EOS, pointing out large discrepancies. In addition, we show that the effect of oxygen decreases 300-K isotherm pressure by 10%-15%. The implication of these observations is the ability to improve ICF target performance, which is essential to achieve fusion ignition.

AIMD Results for a Concentrated Solution of Tetra-ethylammonium Tetra-fluoroborate in Propylene Carbonate

Ab initio molecular dynamics (AIMD) calculations are implemented for a concentrated solution of tetra-ethylammonium tetra-fluoroborate in propylene carbonate (TEABF4/PC), which is of interest in electrochemical double-layer capacitors. AIMD calculations predict more sharply structured ion-ion center partial radial distributions than either GAFF or OPLS-aa force-fields in classic FFMD calculations. This may be due to AIMD refinement of van der Waals interactions which drive ion-pairing and phase separation of TEABF4 from PC at this elevated concentration. Quantifying the charge transfer of BF4 - and N(CH2CH3)4 + ions to neighboring solution species, the (Bader) partial charges of BF4 - and N(CH2CH3)4 + ions in PC solution differ from the formal charges by being smaller in magnitude by about 5%.

Molecular Dynamics Study for Lithium Ion Diffusion in Layered LixCoO2 (x=0.5~1.0)

Diffusion constant of lithium ion is one of the most important properties in cathode materials for lithium ion battery. Various experimental and theoretical approaches have been studied to investigate the diffusion constant in LiCoO2 [1-6]. Lithium vacancy ordering is essential phenomenon to determine the lithium ion dynamics [7, 8] and it is important to understand the relationship between the ordered phase generation and the diffusion constant. In theoretical approach, ab initio calculations using density functional theory (DFT) have been established to evaluate the lithium ion diffusivity in cathodes from lithium hopping model based on activation barrier [4-6]. However, DFT is not appropriate for large simulation including various vacancy configurations because of computational cost explosion. Alternatively, molecular dynamics (MD) is more suitable approach to simulate over ten thousand atoms and MD can directly simulate the lithium diffusion with effective force field. However, generic force field cannot support for variable lithium ion number in LiCoO2 [9-12], because the force field development is difficult under the drastic electronic state changing of cobalt by lithium ion extraction/injection processes. Here, we have improved the force field using embedded atom method to modify the total energies in accordance with amount of neighboring lithium ion. The force field parameters have been optimized to reproduce potential energy surfaces of several lithium diffusion paths estimated by DFT calculations. We also fit vacancy energies and elastic constants at several lithium configurations. In addition, ab initio MD results are used as a reference to reproduce finite temperature fluctuation. For validation, MD simulations in several temperatures were performed. The diffusion constants calculated from mean square displacement of lithium ion were good agreement with experimental results [1, 2] qualitatively. We will talk about microscopic picture of lithium vacancy ordering on that day.

Dissociation and recombination of D₂ on Cu(111): ab initio molecular dynamics calculations and improved analysis of desorption experiments.

Obtaining quantitative agreement between theory and experiment for dissociative adsorption of hydrogen on and associative desorption of hydrogen from Cu(111) remains challenging. Particularly troubling is the fact that theory gives values for the high energy limit to the dissociative adsorption probability that is as much as two times larger than experiment. In the present work we approach this discrepancy in three ways. First, we carry out a new analysis of the raw experimental data for D2 associatively desorbing from Cu(111). We also perform new ab initio molecular dynamics (AIMD) calculations that include effects of surface atom motion. Finally, we simulate time-of-flight (TOF) spectra from the theoretical reaction probability curves and we directly compare them to the raw experimental data. The results show that the use of more flexible functional forms for fitting the raw TOF spectra gives fits that are in slightly better agreement with the raw data and in considerably better agreement with theory, even though the theoretical reaction probabilities still achieve higher values at high energies. The mean absolute error (MAE) for the energy E0 at which the reaction probability equals half the experimental saturation value is now lower than 1 kcal/mol, the limit that defines chemical accuracy, while a MAE of 1.5 kcal/mol was previously obtained. The new AIMD results are only slightly different from the previous static surface results and in slightly better agreement with experiment.

Molecular Structure and Ion Transport Near Electrode-Electrolyte Interfaces in Lithium-Ion Batteries

The performance of lithium-ion secondary batteries (LIBs) is strongly tied to ionic transport through electrode-electrolyte interfaces. Solvation structure strongly affects ion mobility, and differences in ion solvation near the interface compared to the bulk may affect conductivity. Furthermore, the formation and evolution of solid-electrolyte interphase (SEI) layers, which impede transport between electrode and electrolyte while passivating the interface, are related to molecular interactions at the interface involving solvent molecules, ions, and the electrode surface. In addition, the chemistry of the electrode surface, in particular the edge termination of graphite electrodes, can impact Li ion transport into the electrode and formation of SEI. Understanding these effects is critical to optimizing battery performance.Here we present molecular dynamics (MD) simulations of typical organic liquid LIB electrolytes in contact with graphite electrodes to understand the relationship between ion solvation structure and transport near the interface. Results for different graphite terminations are presented and the effect of an electric field is explored. Different prototypical organic liquids, including ethylene carbonate and ethyl methyl carbonate, are compared. We observe that anion mobility in an electric field is much greater than that for Li ions, a phenomenon due to the very different solvation structures. In addition, we observe a barrier to Li insertion into graphite when its surface is hydroxyl terminated. Furthermore, we compare the results of ab initio MD with the empirical reactive forcefield ReaxFF and the non-reactive, non-polarizable COMPASS forcefield and note differences in the predictive power of each technique. Prepared by LLNL under Contract DE-AC52-07NA27344.

Study of translational, librational and intra-molecular motion of adsorbed liquid water monolayers at various TiO2 interfaces

Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the vibrational motion of water in contact with rutile-(110), rutile-(100), rutile-(001), anatase-(101) and anatase-(001) surfaces at room temperature (300 K). The vibrational density of states (VDOS) of the first adsorbed monolayer of liquid water has been analysed for each surface. These have been compared with reported experimental INS values involving rutile and anatase polymorph surfaces, along with ab initio MD results. It is observed that good qualitative agreement is obtained for the rutile-(110) and the anatase-(101) surfaces with the corresponding experimental VDOS. A significant contribution from librational dynamics is found for planar rutile surfaces, but no such demarcation is seen in the experimental VDOS.

Molecular dynamic simulation of the melting process in Ni3Al alloy

The melting process for an intermetallic compound Ni3Al is modeled with the use of two molecular dynamics (MD) methods, i.e., classical MD and ab initio MD. In the first case, we have a good statistics; however the results of modeling significantly depend on the choice of an interatomic interaction potential model. In the second case, any fitting parameters are not necessary, but a great computational power is required. The ab initio MD results are in better agreement with experiment. However, using this method, it is impossible to receive good statistics because of a small number of atoms. The two methods are mutually complimentary and allow to obtain valid results in the modeling of new materials as well.

High-pressure melting of lead

The melting curve of the hexagonal close-packed (hcp) phase of lead (Pb) has been determined over a wide pressure range using both ab initio molecular dynamics (AIMD) simulations and classical molecular dynamics (CMD) employing an effective pair potential. The AIMD simulations are based on a density functional theory (DFT) in the generalized gradient approximation (GGA). The Pb melting curve, constructed using a well-established theoretical scheme, is in excellent agreement with the AIMD results. Our calculated equation of state (EOS) of hcp Pb is in excellent agreement with experimental data up to $40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Our melting curve agrees very well with melting temperatures obtained in both shock-wave and diamond-anvil cell (DAC) experiments, but at higher pressures our curve lies between the two data sets.

Structural changes on supercooling liquid silicon

X-ray diffraction experiments and ab initio molecular dynamics (AIMD) simulations have been performed to investigate the atomic structure of liquid silicon into the deeply supercooled region. The levitation technique used has made it possible to extend the measurements down to a temperature of 1458 K, 230 K below the equilibrium melting point. The x-ray and AIMD results, which are in reasonable agreement, show conclusively that the tetrahedral order is reinforced and that the coordination number decreases as the liquid is supercooled, with implications for the liquid–liquid phase transition.