Performed First-principles Molecular Dynamics Research Articles

Davemaoite as the mantle mineral with the highest melting temperature.

Knowledge of high-pressure melting curves of silicate minerals is critical for modeling the thermal-chemical evolution of rocky planets. However, the melting temperature of davemaoite, the third most abundant mineral in Earth's lower mantle, is still controversial. Here, we investigate the melting curves of two minerals, MgSiO3 bridgmanite and CaSiO3 davemaoite, under their stability field in the mantle by performing first-principles molecular dynamics simulations based on the density functional theory. The melting curve of bridgmanite is in excellent agreement with previous studies, confirming a general consensus on its melting temperature. However, we predict a much higher melting curve of davemaoite than almost all previous estimates. Melting temperature of davemaoite at the pressure of core-mantle boundary (~136 gigapascals) is about 7700(150) K, which is approximately 2000 K higher than that of bridgmanite. The ultrarefractory nature of davemaoite is critical to reconsider many models in the deep planetary interior, for instance, solidification of early magma ocean and geodynamical behavior of mantle rocks.

Structure and Dynamics of NaCl/KCl/CaCl2-EuCln (n = 2, 3) Molten Salts.

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu2+ and Eu3+ ions in CaCl2, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K+ to Na+ to Ca2+, the coordination number (CN) of Cl- in the first solvation shell increases from 5.6 (Eu2+) and 5.9 (Eu3+) in KCl to 6.9 (Eu2+) and 7.0 (Eu3+) in CaCl2. This coordination change is validated by the EXAFS measurements, in which the CN of Cl- around Eu increases from 5 in KCl to 7 in CaCl2. Our simulation shows that the fewer Cl- ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu2+/Eu3+ are related to the rigidity of their first coordination shell of Cl-: the more rigid the first coordination shell is, the slower the solute cations diffuse.

Finite Temperature Ultraviolet-Visible Dielectric Functions of Tantalum Pentoxide: A Combined Spectroscopic Ellipsometry and First-Principles Study

Tantalum pentoxide (Ta2O5) has demonstrated promising applications in gate dielectrics and microwave communication devices with its intrinsically high dielectric constant and low dielectric loss. Although there are numerous studies on the dielectric properties of Ta2O5, few studies have focused on the influence of external environmental changes (i.e., temperature and pressure) on the dielectric properties and the underlying physics is not fully understood. Herein, we synthesize Ta2O5 thin films using the magnetron sputtering method, measure the ultraviolet-visible dielectric function at temperatures varying from 300 to 873 K by spectroscopic ellipsometry (SE), and investigate the temperature influence on the dielectric function from first principles. SE experiments observe that temperature has a nontrivial influence on the ultraviolet-visible dielectric function, accompanying the consistently decreased amplitude and increased broadening width for the dominant absorption peak. First-principles calculations confirm that the dominant absorption peak originates from the aggregated energy states near the valence band maximum (VBM) and conduction band minimum (CBM), and the theoretically predicted dielectric functions demonstrate good agreement with the SE experiments. Moreover, by performing first-principles molecular dynamics simulations, the finite-temperature dielectric function is predicted and its change trend with increasing temperature agrees overall with the SE measurements. This work explores the physical origins of temperature influence on the ultraviolet-visible dielectric function of Ta2O5, aimed at promoting its applications in the field of micro-/nanoelectronics.

Stable polar oxynitrides through epitaxial strain

We investigate energetically favorable structures of $AB{\mathrm{O}}_{2}\mathrm{N}$ oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, density functional theory (DFT) lattice dynamics and first-principles molecular dynamics. We predict several thermodynamically stable polar oxynitride perovskites under high pressures. In addition, we find that ferroelectric polar phases of perovskite-structured oxynitrides can be thermodynamically stable and synthesized at high pressure on appropriate substrates. The dynamical stability of the ferroelectric oxynitrides under epitaxial strain at ambient pressure also implies the possibility to synthesize them using pulsed laser deposition or other atomic layer deposition methods. Our results have broad implications for further exploration of other oxynitride materials as well. We performed first-principles molecular dynamics and find that the polar perovskite of ${\mathrm{YSiO}}_{2}\mathrm{N}$ ($I4cm$) is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonitelike structures ($I4/mcm$). We predict that ${\mathrm{YGeO}}_{2}\mathrm{N}, {\mathrm{LaSiO}}_{2}\mathrm{N}$, and ${\mathrm{LaGeO}}_{2}\mathrm{N}$ are metastable as ferroelectric perovskites ($P4mm$) at zero pressure even without epitaxial strain.

Mechanistic understanding of antimony(V) complexation on montmorillonite surfaces: Insights from first-principles molecular dynamics

Clay minerals play a vital role in retaining antimony (Sb) by surface complexation, which can largely attenuate its mobility and availability. However, a profound understanding of the environmental process of Sb on clay-water interface is insufficient. In this study, aiming to quantitatively revealing the complexation mechanism of Sb(V) onto montmorillonite at the molecular scale, we performed first-principles molecular dynamics (FPMD) simulations to systematically investigate the related structural and energetic information. In interlayer space, [Sb(OH)6]- anion can bind with Ca2+/Na+/Al3+/Fe3+ as stable inner-sphere complexes, and the Sb-Al/Fe complex is much more favorable than Sb-Na/Ca complex. On edge surfaces, the ligand exchange processes on possible sites in a wide range of pH have been examined. At (010) edge, [Sb(OH)6]− was adsorbed as bidentate complex on Al-(H2O)(H2O) and Mg-(H2O)(H2O) and monodentate complex on Al-(H2O)(OH), and at (110) edge [Sb(OH)6]− formed monodentate complex on Al-(H2O) and Mg-(H2O). The desorption free energies indicated that the Sb-Al bidentate complex is the most stable and favorable edge species, followed by the Sb-Mg bidentate complex. Overall, this work provides the molecular-scale insights into the fate of antimony in natural environments, and the derived fundamental data contribute to improving the experiment-based surface complexation models (SCM).

Catalytic Effect of Ti or Pt in a Hexagonal Boron Nitride Surface for Capturing CO2

We investigated the effect of doping a hexagonal boron nitride surface (hBN) with Ti or Pt on the adsorption of CO2. We performed first-principles molecular dynamics simulations (FPMD) at atmospheric pressure, and 300 K. Pristine hBN shows no interaction with the CO2 molecule. We allowed the Ti and Pt atoms to interact separately, with either a B-vacancy or an N-vacancy. Both Ti and Pt ended chemisorbed on the surface. The system hBN + Ti always chemisorbed the CO2 molecule. This chemisorption happens in two possible ways. One is without dissociation, and in the other, the molecule breaks in CO and O. However, in the case of the Pt atom as dopant, the resulting system repels the CO2 molecule.

Mechanism and Electronic Perspective of Oxygen Evolution Reactions Catalyzed by [Fe(OTf)2(bpbp)

We performed first-principles molecular dynamics simulations to study the process of water oxidation by the iron-based molecular catalyst [Fe(OTf)2(bpbp)] (OTf = triflouromethanesulfonate, bpbp = N...

Photoinduced Superhydrophilicity of Anatase TiO2 Surface Uncovered by First-Principles Molecular Dynamics.

TiO2 is a prototype of photocatalyst materials. Interfacial water structure is critical to understand the chemical reactivity of TiO2. By performing first-principles molecular dynamics simulations on the TiO2(001)/water interface were performed, we found that the presence of a photogenerated hole at the interface increases the coverage of both the molecular and dissociative water adsorption by increasing the surface acidity and then shapes the layered and ordered water structure by enhancing the interfacial hydrogen bond network. The enhanced attachment of water in contact with the TiO2 surface rationalizes the increase in the intensity of the sum frequency generation spectrum under ultraviolet illumination reported in the experiment. These findings provide a novel interpretation of the electronic effect for the photoinduced hydrophilic conversion of the TiO2 surface at the atomistic level.

Thermal stability of zirconia-doped ceria surfaces: A first-principles molecular dynamics study

Ceria, one of the most important functional rare-earth oxides, is widely used in many technically important industry fields. However, pure ceria is known to have poor thermal stability. Zirconia doping is usually employed to improve its thermal stability and achieve high OSC (oxygen storage capacity) performances under severe conditions. We have performed first-principles molecular dynamics simulations at a series of temperatures and proposed two parameters, i.e. the thermal displacement rate and the phase segregation degree, to describe the structural change of the ceria-zirconia solid solution surfaces. Based on our molecular dynamics simulations the temperature predicted at which the specific surface area dramatically decreases is in good agreement with previous experimental observations.

Dynamics and Spectral Response of Water Molecules around Tetramethylammonium Cation.

Till now, there has been ambiguity about the structural heterogeneity inside a solute solvation shell and the dynamical response of the surrounding solvent molecules. To address the dynamics and spectral response of solvent molecules, we performed first-principles molecular dynamics simulations for the comprehensive study of water's hydroxyl stretch frequency evolution due to environmental variations (also called "spectral diffusion") in the vicinity of a hydrophobe, tetramethylammonium (TMA) cation. The N-Ow radial distribution function (RDF), spatial distribution function (SDF), and combined distribution function (CDF) were calculated to provide information about the arrangement of water molecules around TMA. In the probability distribution plot of the cosine of the angle (θ) between Ow-Hw and NTMA-Ow bond vectors, the hydrogen atoms are observed being oriented toward TMA and the oxygen atoms aligned away. The decaying dynamics of the orientation autocorrelation function (OACF) reveals the reorientation time is more inside the solvation shell (4.1 ps) as compared to bulk (2.8 ps), matching with the trend obtained from water's orientational dynamics in tetraalkylammonium salts. Wavelet transform of the obtained trajectory was used to calculate the time-dependent vibrational stretching frequencies of the OH modes of water molecules. The normalized frequency distribution in the aqueous solvation shell of TMA, tagging a particular water molecule within the N-Ow cutoff distance 5.5 Å, displays an intense peak at 3661 cm-1 representing non-hydrogen bonded or dangling or free OH modes. Simulations around aromatic solutes and Raman-MCR studies in the hydration shell of hydrophobic TBA reported a distinctive dangling OH band at 3660 cm-1 (range: 3661 ± 2 cm-1). Besides dangling water molecules in the first hydration layer of ammonium nitrogen, few OH modes are strongly hydrogen-bonded having an average frequency at 3300 cm-1. The predominance of dangling hydroxyl modes around apolar hydrophobic TMA was further explored by comparing the dangling lifetime (∼0.68 ps) with the lifespan of hydrogen bonded OH modes (∼0.48 ps). At our simulation temperature 330 K, a significant fraction of the water molecules in the vicinity of TMA ion are free or dangling, and a few of them form an ordered structure with enhanced hydrogen bonding. Structural analysis, orientation correlation, frequency fluctuations, dangling, and hole dynamics calculations provide the evidence of the existence of dangling OH modes dominating over highly ordered strong hydrogen bonded structure in the cationic TMA solvation shell at an elevated temperature.

Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules.

We performed first-principles molecular dynamics simulations of relatively dilute aqueous solutions of sulfate and thiosulfate dianions to analyze the structure, dynamics, and vibrational spectral properties of water molecules around the solute, especially the spatially patterned solvent molecules in the first solvation layer and the extended layers. This study also involves the investigation of dynamics of dangling OH groups in these layers and their role in patterning the water molecules around the dianions. Structural evaluation of the systems is carried out by radial distribution functions, number integrals, and spatial distribution functions. The lifetime of dangling OH groups inside the solvation shell is compared more to that of the bulk. By constructing the O-H groups in three ensembles (S1, S2, and S3) around the anion, we show that the frequency distribution of OH modes in the S1 ensemble show red-shifting for both sulfate and thiosulfate. The O-H groups in the S2 ensemble of the sulfate-water system show red-shifting by 10 cm-1, while in the case of thiosulfate-water, these O-H groups show blue-shifting by 8 cm-1. The water molecules in S1 and S2 subensembles have slower dynamics compared to those in the bulk (S3). The dynamics of various kinds of hydrogen bonds were characterized by hydrogen bond population correlation functions. The spectral diffusion of solvation shell O-H modes was performed through a frequency-time correlation function. We find a significant amount of orientational retardation of water molecules in the S1 layer and moderate retardation in the S2 layer as compared to that in the bulk, S3 layer. All these findings, the red shift of the OH stretching frequency in S1 and S2 layers, slowing down of the orientational dynamics of OH vectors in S1 and S2 layers, and less diffusivity of water in S1 and S2 layers, show the long-range kosmotropic effect of multivalent sulfate and thiosulfate oxyanions. Due to the long-range effect, heterogeneous occupancy of water molecules is observed, and the water molecules are found to arrange in a patterned manner in the vicinity of anions with varied local density.

Repeatable and reproducible formation/rupture of oxygen vacancy filaments in the vicinity of a polycrystalline HfO2 surface

We theoretically investigated the dynamics of oxygen vacancies (Vo’s) and the effect of changing their charge states by performing first-principles molecular dynamics simulations at a temperature of 1000 K. Calculations were performed for two structures of HfO2: a slab with (110) surfaces and a bulk single crystal. Our studies revealed that when the charge state of the Vo’s changed from neutral (VO0) to divalent (VO2+), Vo’s repelled each other and dispersed, in both the slab and bulk structures. In contrast, when the charge state of the Vo’s changed from VO2+ to VO0, the Vo’s attracted each other and resumed their original positions only in the slab structure. Therefore, the repeatable and reproducible formation/rupture of a VO filament can occur near the crystal surface, where the symmetry of the bulk crystal is broken. This result is consistent with the experimental results demonstrating that the resistive switching of the resistive random access memory develops in polycrystalline metal oxides rather than in single crystalline metal oxides.

Effect of Water Adsorption on the Interfacial Structure and Band Edge Alignment of Anatase TiO2(001)/Water by First-Principles Molecular Dynamics

The generation of hydrogen by photoelectrochemical (PEC) water splitting is a promising pathway to produce renewable and scalable carbon-free energy. To improve the performance of PEC cells, it is essential to understand the structural and electronic properties of the photoelectrode/electrolyte heterogeneous interfaces. By performing first-principles molecular dynamics (FPMD) simulations, we revealed the dynamic structure, such as water dissociation and adsorption and hydrogen bond network, and the band edge alignment of the TiO2/water interfaces. Mixed molecular and dissociative water adsorption is adopted at the (001)/water interface with a water dissociation percentage of ∼25% stabilized at ambient conditions, in contrast to the unstable water dissociation at the (101)/water interface. The different interfacial structures for the two surfaces lead to their distinct proton transfer processes. The calculated band edge alignment across the interfaces shows good agreement with the experimental data and exp...

First Principles Study on Hydrogen Intercalation into Buffer Layer Grown on SiC(0001) Surface

Graphene has attracted much attention because it has quite high electron mobility due to the characteristic electronic properties such as Dirac cone and then has been expected as a future electronic device material to contribute to the low energy consumption, which could help combat the global warming. Among graphene fabrication methods suggested so far, thermal decomposition of SiC substrate, in which graphene sheets are formed after Si atom sublimation from SiC surface at high temperature, has been intensively studied. The C-atom layer directly grown on the SiC substrate is not graphene but so-called buffer layer (BL), which has similar honeycomb structure to graphene but does not have the graphene's characteristic electronic properties such as the Dirac cone, because BL is covalently bonded to the SiC surface and lacks the hexagonal symmetry. To utilize BL grown on SiC substrate as graphene, it is necessary to anneal it under hydrogen (H) ambient to intercalate H atoms between BL and SiC substrate, so that the graphene’s characteristic properties is recovered. However, the lack of knowledge on the intercalation mechanism make it difficult to control this process to obtain high quality graphene. In this paper, we report our recent studies on the interaction between H atoms and BL grown on SiC substrate with the periodicity of (6√3x6√3)R30˚ by using first-principles density functional calculations [1]. We first investigated the adsorption process of H2 molecule on BL. The total adsorption energy for two H atoms is 1.6 eV, then H2 molecules prefer to dissociatively adsorb on BL. The activation energy of this process is 0.9 eV and that of the reverse process is 2.5 eV. Considering the fact that H atoms on Si(001) surface desorb from the surface at around 780K with the activation energy of about 2.5eV, we conjecture that H atoms on BL would also desorb from it at the temperature at which the experiments are conducted (above 870K). The H atom adsorption energy ranges between 0.94 eV and -0.35 eV depending on the adsorption sites. Adsorption energies are evaluated based on a H2 molecule in vacuum, and the positive values mean stable states, while negative ones mean unstable states. This variety comes from the C atom bonding states. On the C atom with π bonds, H atom is quite stable with a large adsorption energy, while on the C atom without π bonds due to the bonding to a substrate Si atom, H atom is not stable (sometimes it is endothermic). For the H atom diffusion process, it is found that the activation energy ranges from 1.7 eV to 2.3 eV depending on the diffusion path. This variety comes from the initial and the final states of H atom. If the initial and the final states are stable (on C atom with π bonds), the activation barrier is small like 1.7 eV, while if one of the two is unstable (on C atom without π bonds), the activation energy is large like 2.3 eV. We also investigate the penetration process of H atoms through BL. In the final state, H atom stays on a Si atom of the SiC substrate below BL. The final state is 0.8 eV higher in energy than the initial state where H atom stays on BL. So, H atoms prefers to stay on BL rather than penetrate through BL. The activation barrier of this penetration process is about 4.9 eV, which is quite larger than that of H2 desorption process described above, meaning that H atoms prefer to desorb from BL rather than penetrate through BL. We also study the behaviour of H atoms or molecules below BL. It is found that H molecules put between BL and SiC surfaces easily dissociate into two H atoms, which then make covalent bonds to Si dangling bonds. Surprisingly, there is no energy barrier for the adsorption of H molecule onto Si dangling bonds on a SiC(0001) surface. This is quite contrastive to the adsorption of H molecule onto BL. To investigate the diffusion of H atoms below BL, we performed first-principles molecular dynamics simulation in NVT condition (T=1500K). Even in a short simulation time of three picoseconds, we observed several H atom hoppings from one Si dangling bond to another, meaning that H atom easily diffuse below BL. Acknowledgments: This research was partially supported by MEXT within the priority issue 6 of the FLAGSHIP2020. Earth Simulator of JAMSTEC and NIMS Numerical Materials Simulator were used for this study. [1] PHASE code: https://azuma.nims.go.jp/.

Complex Ion Dynamics in Carbonate Lithium-Ion Battery Electrolytes

Li-ion battery performance is strongly influenced by ionic conductivity, which depends on the mobility of the Li ions in solution, and is related to their solvation structure. In this work, we have performed first-principles molecular dynamics (FPMD) simulations of a LiPF6 salt solvated in different Li-ion battery organic electrolytes. We employ an analytical method using relative angles from successive time intervals to characterize complex ionic motion in multiple dimensions from our FPMD simulations. We find different characteristics of ionic motion on different time scales. We find that the Li ion exhibits a strong caging effect due to its strong solvation structure, while the counterion, PF6– undergoes more Brownian-like motion. Our results show that ionic motion can be far from purely diffusive and provide a quantitative characterization of the microscopic motion of ions over different time scales.

Chemical states of 3d transition metal impurities in a liquid lead-bismuth eutectic analyzed using first principles calculations.

Steels are easily corroded in a liquid lead-bismuth eutectic (LBE) because their components, such as Fe, Cr and Ni, exhibit a high solubility in the liquid LBE. To understand the reason for such a high solubility of these 3d transition metals, we have performed first-principles molecular dynamics calculations and analyzed the pair-correlation functions, electronic densities of states, and Bader charges and volumes of the 3d transition metals dissolved in the liquid LBE as impurities. The calculations show that the 4s and 3d orbitals of the 3d impurity atoms largely interact with the 6p band of the LBE, which generates bonding orbitals. We suggest that the high stability of 3d metals in the liquid LBE is caused by the interactions of the 4s and 3d orbitals with the 6p band. Spin polarization is induced by V, Cr, Mn, Fe and Co impurity atoms in a similar manner to the Slater-Pauling curve of solid transition metals, which exhibits a downward shift in the atomic number by approximately two. Based on the degree of spin polarization and the shifted trend of the Slater-Pauling curve, we suggest that Ni exhibits a higher solubility than Cr and Fe because of the differences in their interaction strengths between their 3d orbitals and the 6p band. In addition, the 4s and 3d orbitals of the 3d impurity atoms were found to interact more favorably with the Bi 6p band than the Pb 6p band, which is consistent with the fact that liquid Bi is more corrosive to steels than is liquid Pb.

Theoretical Investigation on the Behavior of Tetrathiafulvalene (TTF) As a Red-Ox Mediator for Charge Process of Li-Air Battery

Li-Air battery have recently engaged much attention as a next generation energy-storage device because of its high theoretical specific energy. Under the environment of non-aqueous solvent, Li2O2 (Li peroxide) is formed as discharge product covering the porous cathode. In order to make a Li-Air battery rechargeable, this Li2O2 should be decomposed into Li+ and O2 during the charge process. When Li2O2 is directly attached to the cathode surface, oxidation of Li2O2 undergoes by giving electron to the cathode. Whereas, TEM observation implies that such a contact is lost at an early stage of charging. Once Li2O2 is detached from the cathode surface, it becomes hard for these particles to give electron to the cathode, and thus further decomposition is suppressed. This is the origin of charging overpotential and alternative decomposition of electrolyte molecules could proceed. It is known that tetrathiafulvalene (TTF) can reduce the overpotential and promote the decomposition of Li2O2 particles with the combination of DMSO solvent and nano-porous gold electrode, and improvement of the cycle stability as a Li-Air battery is drastically achieved. To clarify the role of TTF as a redox mediator from a microscopic point of view, we systematically performed first-principles molecular dynamics (MD) simulations and elucidated how electron transfer occurs in this system, regarding decomposition of Li2O2. We prepared a model system which contains a cluster of Li2O2 (four atoms model) or Li3O2 + (five atoms model; another stable form of Li2O2) with/without DMSO solvent. The excess charge given to a Li2O2 cluster is localized while that given to a Li2O2 slab is delocalized. This means that the oxidation of Li2O2 becomes local with decrease in the cluster size. Once Li2O2 is partially converted to LiO2 by such a local oxidation, the LiO2 part is spontaneously decomposed into Li+ and O2 − around the positively biased cathode, where O2 − is oxidized to O2. These above are the reason why we treat only the smallest clusters of Li peroxide, and the objective is translated to evaluation of the charge transfer between {Li2O2 or Li3O2 +} and {O2 or TTF+} in DMSO solvent or vacuum. The first-principles MD simulation was performed at 400K using a 1.7 nm cubic cell with 20-23 DMSO molecules. GGA/PBE functional was used, and we give total charge of +2 for TTF/Li3O2 and +1 for TTF/Li2O2 systems, respectively. From Bader charge analysis after 100 ns simulations, we found the charge state of TTF becomes almost neutral in both the Li2O2 and Li3O2 systems with DMSO solvent. This indicates that TTF can oxidize the two clusters. The O-O bond length, which clearly changes reflecting the difference of charge state among O2, O2 −, and O2 2-, also supported the trend of charge. In vacuum condition (without DMSO solvent), TTF becomes positive. This implies DMSO molecules promote the oxidation of Li2O2 (or Li3O2) by TTF. On the other hand, while the charge state of O2 in the Li2O2 system approaches -1, that in the Li3O2 system becomes almost neutral. This means that O2 can oxidize and decompose Li2O2, but it is not easy for O2 to decompose Li3O2 + in DMSO. Hence, we obtained the picture that O2 molecule oxidizes and decomposes most of the Li peroxide clusters during the charge process (e.g. O2 − poor environment) and TTF+ plays a critical role in the decomposition of Li3O2 suppressing overpotential and prevent their accumulation in the final stage. We will discuss this scenario in the presentation for more details.

First-principles simulations of CaO and CaSiO3 liquids: structure, thermodynamics and diffusion

We have performed first-principles molecular dynamics simulations of CaO and CaSiO3 liquids over broad ranges of pressure (0–150 GPa) and temperature (2,500–8,000 K) within density-functional theory. The simulated liquid structure changes considerably on compression with the mean cation–anion coordination numbers increasing nearly linearly with volume. The Ca–O coordination number increases from 5 (7) near the ambient pressure to 8 (10) at high pressure for CaO (CaSiO3) liquid. The Si–O coordination number increases from 4 to 6 over the same pressure regime. Our results show that both liquids are much more compressible than their solid counterparts implying the possibility of liquid–solid density crossovers at high pressure. The Gruneisen parameter of both the liquids increases with pressure, which is opposite in case of crystalline phases. The calculated self-diffusion coefficients strongly depend on temperature and pressure, thereby requiring non-Arrhenian representation with variable activation volume. The diffusivity differences between the two liquids tend to be large at low-temperature and low-pressure regime. Also, comparisons with MgSiO3 liquid suggest that network modifier cations Ca and Mg behave similarly though Ca is more coordinated and more mobile as compared to Mg.

Experimental and Theoretical Study of Multi-Quantum Vibrational Excitation: NO(v = 0→1,2,3) in Collisions with Au(111)

We measured absolute probabilities for vibrational excitation of NO(v = 0) molecules in collisions with a Au(111) surface at an incidence energy of translation of 0.4 eV and surface temperatures between 300 and 1100 K. In addition to previously reported excitation to v = 1 and v = 2, we observed excitation to v = 3. The excitation probabilities exhibit an Arrhenius dependence on surface temperature, indicating that the dominant excitation mechanism is nonadiabatic coupling to electron-hole pairs. The experimental data are analyzed in terms of a recently introduced kinetic model, which was extended to include four vibrational states. We describe a subpopulation decomposition of the kinetic model, which allows us to examine vibrational population transfer pathways. The analysis indicates that sequential pathways (v = 0 → 1 → 2 and v = 0 → 1 → 2 → 3) alone cannot adequately describe production of v = 2 or 3. In addition, we performed first-principles molecular dynamics calculations that incorporate electronically nonadiabatic dynamics via an independent electron surface hopping (IESH) algorithm, which requires as input an ab initio potential energy hypersurface (PES) and nonadiabatic coupling matrix elements, both obtained from density functional theory (DFT). While the IESH-based simulations reproduce the v = 1 data well, they slightly underestimate the excitation probabilities for v = 2, and they significantly underestimate those for v = 3. Furthermore, this implementation of IESH appears to overestimate the importance of sequential energy transfer pathways. We make several suggestions concerning ways to improve this IESH-based model.

First-Principles Molecular Dynamics at a Constant Electrode Potential

A simulation scheme for performing first-principles molecular dynamics at a constant electrode potential is presented, opening the way for a more realistic modeling of voltage-driven devices. The system is allowed to exchange electrons with a reservoir at fixed potential, and dynamical equations for the total electronic charge are derived by using the potential energy of the extended system. In combination with a thermostat, this potentiostat scheme reproduces thermal fluctuations of the charge with the correct statistics, implying a realistic treatment of the potential as a control variable. Practically, the dynamics of the charge are decoupled from the electronic structure calculations, making the scheme easily implementable in existing first-principles molecular dynamics codes. Our approach is demonstrated on a test system by considering various test cases.