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Accurate Force Fields (FFs) are essential for Molecular Dynamics (MD) simulations of the dynamics of realistic materials in terms of atomic-level interactions. The FF parameters of short-range valence interactions can be derived through Quantum Mechanical (QM) calculations on model systems practical for QM (<300 atoms). Similarly, the dynamic electrostatic interactions can be described with methods such as QEq or PQEq that allow charges and polarization to adjust dynamically. However, accurately extracting long-range van der Waals (vdW) interactions from QM calculations poses challenges due to the absence of a definitive method to distinguish between the different energetic components of electrostatics, polarization, vdW, hydrogen bonding, and valence interactions. To do this we use the Perdew–Burke–Ernzerhof flavor of Density Functional Theory, including empirical D3 vdW corrections, to predict the Equation of State for each element (keeping any covalent bonds fixed), from which we obtain the two-body vdW nonbond potential. Here, we extend these calculations to include non-bonded parameters for the N and O columns of the periodic table so that we now describe columns 15 (N), 16 (O), 17 (F), and 18 (Ne) of the periodic table. For these 20 elements, we find that the two-body vdW potentials can all be mapped to a single universal two-body curve, with just three scaling parameters: Re, De, and L. We refer to this as the Universal NonBond (UNB) potential. We expect this to be useful for new MD simulations and a helpful starting point to obtain UNB parameters for the remainder of the periodic table. [ABSTRACT FROM AUTHOR]

We outline the recently developed Grand Canonical Potential Kinetics (GCP-K) method to implement Grand Canonical Quantum Mechanics for predicting electrochemical reactions as a function of applied potential rather than with fixed numbers of electrons as in traditional Quantum Mechanics (QM) calculations. We describe here the recent validation of GCP-K for The Co/TiO2 single atom catalyst for the Oxygen Evolution Reaction (OER) on a single crystal nanoparticle where a single surface facet was present. The basal plane of transition metal dichalcogenides (TMDs) in the 1 T' phase of WSe2 and WTe2, which leads to high-performance for the hydrogen evolution reaction (HER). We find that GCP-K predicts accurate TOF and currents as a function of applied potential and accurate Tafel slopes for both the Co/TiO2 OER and the chalcogenide HER systems for which we can be confident of the surface structure. Thus we expect that these methods will be useful in the more common situation for which there is much less certainty about the surface structure under experimental conditions. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations require accurate force fields (FFs) to describe the physical and chemical properties of complex materials and systems. FF parameters for valence interactions can be determined from high-quality Quantum Mechanical (QM) calculations. However, it has been challenging to extract long-range nonbonded interaction potentials from QM calculations since there is no unambiguous method to separate the total QM energy into electrostatics (polarization), van der Waals (vdW), and other components. Here, we propose to use density functional theory with dispersion corrections to obtain the equation of state for single element solid systems (of H, C, N, O, F, Cl, Br, I, P, He, Ne, Ar, Kr, Xe, and Rn) from which we obtain the pure 2-body vdW nonbonded potentials. Recently, we developed the polarizable charge equilibration (PQEq) model based on QM polarization energy of electric probe dipoles with no contributions from vdW. Together, the vdW and PQEq interactions form the nonbonded potential of our new transferrable reactive FF (RexPoN). They may also be useful to replace the nonbonded parts of standard FFs, such as OPLS, Amber, UFF, and CHARMM. We find that the individual 2-body vdW potential curves can be scaled to a universal vdW potential using just three specific atomic parameters. This simplifies extension to the rest of the periodic table for atoms that do not exhibit molecular packing. We validate the accuracy of these nonbonded interactions for liquid water, energetic, and biological systems. In all cases, we find that our new nonbonded potentials provide good agreement with QM and experimental data. [ABSTRACT FROM AUTHOR]

The industrial pollutant NO is a potential threat to the environment and to human health. Thus, selective catalytic reduction of NO into harmless N2, NH3, and/or N2O gas is of great interest. Among many catalysts, metal Pd has been demonstrated to be most efficient for selectivity of reducing NO to N2. However, the reduction mechanism of NO on Pd, especially the route of N−N bond formation, remains unclear, impeding the development of new, improved catalysts. We report here the elementary reaction steps in the reaction pathway of reducing NO to NH3, N2O, and N2, based on density functional theory (DFT)-based quantum mechanics calculations. We show that the formation of N2O proceeds through an Eley-Rideal (E−R) reaction pathway that couples one adsorbed NO* with one non−adsorbed NO from the solvent or gas phase. This reaction requires high NO* surface coverage, leading first to the formation of the trans-(NO)2* intermediate with a low N−N coupling barrier (0.58 eV). Notably, trans-(NO)2* will continue to react with NO in the solvent to form N2O, that has not been reported. With the consumption of NO and the formation of N2O* in the solvent, the Langmuir-Hinshelwood (L-H) mechanism will dominate at this time, and N2O* will be reduced by hydrogenation at a low chemical barrier (0.42 eV) to form N2. In contrast, NH3 is completely formed by the L-H reaction, which has a higher chemical barrier (0.87 eV). Our predicted E-R reaction has not previously been reported, but it explains some existing experimental observations. In addition, we examine how catalyst activity might be improved by doping a single metal atom (M) at the NO* adsorption site to form M/Pd and show its influence on the barrier for forming the N−N bond to provide control over the product distribution. [ABSTRACT FROM AUTHOR]

We report here a new force field for water based solely on quantum mechanics (QM) calculations with no empirical data. The QM was at a high level, coupled cluster single double triple, for all orientations and distances for water dimer plus X3LYP density functional theory (DFT) on 19 larger water clusters. In addition, we included charge and polarization based on the polarizable charge equilibration method and nonbond interactions from DFT-D3 calculations on the H2 and O2 crystal. This model, denoted as RexPoN, provides quite excellent agreement with experimental (expr) data for the solid and liquid phase of water: Tmelt= 273.3 K (expr = 273.15 K) and properties at 298 K: ΔHvap = 10.36 kcal/mol (expr = 10.52), density = 0.9965 gr/cm3 (expr = 0.9965), entropy = 68.4 (J/mol)/K (expr = 69.9), dielectric constant = 76.1 (expr = 78.4), and ln Ds (self-diffusion coef) = −10.08 (expr = −11.24). Such an accurate force field for water will, we believe, be useful for full solvent calculations of electrocatalysis, where we can restrict QM water to just the first one or two layers involving reactions, using RexPoN to provide the polarization for a more distant solvent. Also, RexPoN may provide a better description of the solvent for proteins, DNA, polymers, and inorganic systems for applications to biomolecular, pharma, electrocatalysis (fuel cells and water splitting), and batteries where interaction with explicit water molecules plays a significant role. [ABSTRACT FROM AUTHOR]

First-principles calculations combining density-functional theory and continuum solvation models enable realistic theoretical modeling and design of electrochemical systems. When a reaction proceeds in such systems, the number of electrons in the portion of the system treated quantum mechanically changes continuously, with a balancing charge appearing in the continuum electrolyte. A grand-canonical ensemble of electrons at a chemical potential set by the electrode potential is therefore the ideal description of such systems that directly mimics the experimental condition. We present two distinct algorithms: a self-consistent field method and a direct variational free energy minimization method using auxiliary Hamiltonians (GC-AuxH), to solve the Kohn-Sham equations of electronic density-functional theory directly in the grand canonical ensemble at fixed potential. Both methods substantially improve performance compared to a sequence of conventional fixed-number calculations targeting the desired potential, with the GC-AuxH method additionally exhibiting reliable and smooth exponential convergence of the grand free energy. Finally, we apply grand-canonical density-functional theory to the under-potential deposition of copper on platinum from chloride-containing electrolytes and show that chloride desorption, not partial copper monolayer formation, is responsible for the second voltammetric peak. [ABSTRACT FROM AUTHOR]

Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film. [ABSTRACT FROM AUTHOR]

• The Grasselli team at SOHIO developed and optimized the SOHIO multicomponent catalyst for ammoxidation of propene and ammonia to acrylonitrile. • The Grasselli team deduced mechanistic oncepts from their experiments for guidance in optimization, but later accepted the Goddard mechanism. • This stimulated Goddard at Caltech to use Quantum Mechanics to provide a new mechanistic basis to explain atomistic details of mechanism. This paper is based on the talk given by Goddard at the symposium "New Vistas in Heterogeneous Catalysis: Symposium in honor of Robert Grasselli" sponsored by the CATL division of the ACS held on August 21, 2018 at the ACS national meeting in Boston MA. It tells the story about how Bob Grasselli had a dramatic influence on the research I have carried out on heterogeneous catalysis, particularly on ammoxidation. [ABSTRACT FROM AUTHOR]

Electrocatalysis may provide the solution to some of the most important energy and environmental problems facing society: converting solar energy during the day to fuel (H2) that can provide power at night (hydrogen fuel cells) through water splitting, ·reducing the CO2 in the atmosphere to valuable chemicals (methane, ethylene, ethanol). However significant improvements must be made in the selectivity and activity of current electrocatalysts to obtain practical solutions. A great many experiments are underway to find such solutions, but the progress is slow. We consider that quantum mechanics based multiscale simulations can dramatically accelerate the progress by identifying the reaction mechanisms involved and the using in silico methods to predict the best modifications to Improve performance. We will discuss some of the progress in developing the methods needed and applying them to improving electrocatalysts. [ABSTRACT FROM AUTHOR]

In the oxidative dehydrogenation (ODH) of alkanes, some important advances of the last decade have made it possible to accelerate the development and industrial insertion of the M1 based catalytic systems. These catalysts may be fine-tuned to account for the inevitable variability of different chemicals and impurities in the feedstocks. The latter may include, among others, different blends of shale gases, with different ratios of the C1-C3 alkanes and impurities such as sulfur, phosphorus, etc. In this article, we review the recent progress achieved in our understanding of the crystal structures and the oxidative dehydrogenation (ODH) reaction mechanisms of the multi-metal oxide (MMO) M1 catalyst. Firstly, the complex crystal structure of the M1 phases has been examined using quantum mechanics (QM), reactive force field (ReaxFF), and machine learning (ML) approaches. Secondly, we discussed the ODH mechanism on the M1 phase based on the QM simulations including the finite cluster model and the periodic slab model. Finally, we proposed a catalyst design approach to improve the selectivity of the M1 phase based upon the ODH reaction mechanism. We also briefly discuss the concept of the CE ("Concurrent Engineering", introduced by the European Space Agency). The development of the CE concepts may be applied to the M1 catalytic systems in the future allowing businesses to be agile and react fast to the changing production conditions, thereby making them uniquely competitive in the ODH of alkanes and other areas. [ABSTRACT FROM AUTHOR]

Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally. Here, we apply the recently developed grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential (U) to determine faradic efficiency, turn over frequency, and Tafel slope for CO and H2 production for all three sites. We predict an onset potential (at 10 mA cm−2) Uonset = −0.84 V (vs. RHE) for Ni-N2C2 site and Uonset = −0.92 V for Ni-N3C1 site in agreement with experiments, and Uonset = −1.03 V for Ni-N4. We predict that the highest current is for Ni-N4, leading to 700 mA cm−2 at U = −1.12 V. To help determine the actual sites in the experiments, we predict the XPS binding energy shift and CO vibrational frequency for each site. Single atom catalysts (SACs) are promising in electrocatalysis but challenging to characterize. Here, the authors apply a recently developed quantum mechanical grand canonical potential kinetics method to predict reaction mechanisms and rates for CO2 reduction at different sites of graphene-supported Ni-SACs. [ABSTRACT FROM AUTHOR]

This issue of PNAS features "nonequilibrium transport and mixing across interfaces," with several papers describing the nonequilibrium coupling of transport at interfaces, including mesoscopic and macroscopic dynamics in fluids, plasma, and other materials over scales from microscale to celestial. Most such descriptions describe the materials in terms of the density and equations of state rather than specific atomic structures and chemical processes. It is at interfacial boundaries where such atomistic information is most relevant. However, there is not yet a practical way to couple these phenomena with the atomistic description of chemistry. The starting point for including such information is the quantum mechanics (QM). However, practical QM calculations are limited to a hundred atoms for dozens of picoseconds, far from the scales required to inform the continuum level with the proper atomistic description. To bridge this enormous gap, we need to develop practical methods to extend the scale of the atomistic simulation by several orders of magnitude while retaining the level of QM accuracy in describing the chemical process. These developments would enable continuum modeling of turbulent transport at interfaces to incorporate the relevant chemistry. In this perspective, we will focus on recent progress in accomplishing these extensions in first principles-based atomistic simulations and the strategies being pursued to increase the accuracy of very large scales while dramatically decreasing the computational effort. [ABSTRACT FROM AUTHOR]

Electrocatalysis provides a powerful means to selectively transform molecules, but a serious impediment in making rapid progress is the lack of a molecular-based understanding of the reactive mechanisms or intermediates at the electrode-electrolyte interface (EEI). Recent experimental techniques have been developed for operando identification of reaction intermediates using surface infrared (IR) and Raman spectroscopy. However, large noises in the experimental spectrum pose great challenges in resolving the atomistic structures of reactive intermediates. To provide an interpretation of these experimental studies and target for additional studies, we report the results from quantum mechanics molecular dynamics (QM-MD) with explicit consideration of solvent, electrode-electrolyte interface, and applied potential at 298 K, which conceptually resemble the operando experimental condition, leading to a prototype of operandoQM-MD(o-QM-MD). With o-QM-MD, we characterize 22 possible reactive intermediates in carbon dioxide reduction reactions (CO2RRs). Furthermore, we report the vibrational density of states (v-DoSs) of these intermediates from two-phase thermodynamic (2PT) analysis. Accordingly, we identify important intermediates such as chemisorbed CO2 (b-CO2), *HOC-COH, *C-CH, and *C-COH in our o-QM-MD likely to explain the experimental spectrum. Indeed, weassign the experimentalpeakat 1,191cm-1 to themodeofC-Ostretch in*HOC-COH predicted at 1,189 cm-1 and the experimental peak at 1,584 cm-1 to the mode of C-C stretch in *C-COD predicted at 1,581 cm-1. Interestingly, we find that surface ketene (*C=C=O), arising from *HOC-COH dehydration, also shows signals at around 1,584 cm-1, which indicates a nonelectrochemical pathway of hydrocarbon formation at low overpotential and high pH conditions. [ABSTRACT FROM AUTHOR]

We developed the RexPoN force field for water based entirely on quantum mechanics. It predicts the properties of water extremely accurately, with Tmelt = 273.3 K (273.15 K) and properties at 298 K: ΔHvap = 10.36 kcal/mol (10.52), density = 0.9965 g/cm3 (0.9965), entropy = 68.4 J/mol/K (69.9), and dielectric constant = 76.1 (78.4), where experimental values are in parentheses. Upon heating from 0.0 K (ice) to 273.0 K (still ice), the average number of strong hydrogen bonds (SHBs, rOO = 2.93 Å) decreases from 4.0 to 3.3, but upon melting at 273.5 K, the number of SHBs drops suddenly to 2.3, decreasing slowly to 2.1 at 298 K and 1.6 at 400 K. The lifetime of the SHBs is 90.3 fs at 298 K, increasing monotonically for lower temperature. These SHBs connect to form multibranched polymer chains (151 H2O per chain at 298 K), where branch points have 3 SHBs and termination points have 1 SHB. This dynamic fluctuating branched polymer view of water provides a dramatically modified paradigm for understanding the properties of water. It may explain the 20-nm angular correlation lengths at 298 K and the critical point at 227 K in supercooled water. Indeed, the 15% jump in the SHB lifetime at 227 K suggests that the supercooled critical point may correspond to a phase transition temperature of the dynamic polymer structure. This paradigm for water could have a significant impact on the properties for protein, DNA, and other materials in aqueous media. [ABSTRACT FROM AUTHOR]

The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide (B4C) hard ceramic, this theoretical shear strength can be exceeded by 11% by imposing nanoscale twins. We also predict from QM that the indentation strength of nanotwinned B4C is 12% higher than that of the perfect crystal. Further, we validate this effect experimentally, showing that nanotwinned samples are harder by 2.3% than the twin-free counterpart of B4C. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nanotwins due to the directional nature of covalent bonds at the TB. [ABSTRACT FROM AUTHOR]

Findings of laser-assisted atom probe tomography experiments on boron carbide elucidate an approach for characterizing the atomic structure and interatomic bonding of molecules associated with extraordinary structural stability. The discovery of crystallographic planes in these boron carbide datasets substantiates that crystallinity is maintained to the point of field evaporation, and characterization of individual ionization events gives unexpected evidence of the destruction of individual icosahedra. Statistical analyses of the ions created during the field evaporation process have been used to deduce relative atomic bond strengths and show that the icosahedra in boron carbide are not as stable as anticipated. Combined with quantum mechanics simulations, this result provides insight into the structural instability and amorphization of boron carbide. The temporal, spatial, and compositional information provided by atom probe tomography makes it a unique platform for elucidating the relative stability and interactions of primary building blocks in hierarchically crystalline materials. [ABSTRACT FROM AUTHOR]

It is well known that graphite has a low capacity for Na but a high capacity for other alkali metals. The growing interest in alternative cation batteries beyond Li makes it particularly important to elucidate the origin of this behavior, which is not well understood. In examining this question, we find a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same column of the periodic table. We demonstrate this with quantum mechanics calculations for a wide range of substrate materials (not limited to C) covering a variety of structures and chemical compositions. The phenomenon arises from the competition between trends in the ionization energy and the ion-substrate coupling, down the columns of the periodic table. Consequently, the cathodic voltage for Na and Mg is expected to be lower than those for other metals in the same column. This generality provides a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems. [ABSTRACT FROM AUTHOR]

We used density functional theory quantum mechanics with periodic boundary conditions to determine the atomistic mechanism underlying catalytic activation of propane by the Ml phase of Mo-V-Nb-Te-0 mixed metal oxides. We find that propane is activated by Te--O through our recently established reduction-coupled oxo activation mechanism. More importantly, we find that the C -H activation activity of Te = 0 is controlled by the distribution of nearby V atoms, leading to a range of activation barriers from 34 to 23 kcal/mol. On the basis of the new insight into this mechanism, we propose a synthesis strategy that we expect to form a much more selective single-phase Mo-V-Nb-Te-0 catalyst. [ABSTRACT FROM AUTHOR]

Ceramics are strong, but their low fracture toughness prevents extended engineering applications. In particular, boron carbide (B4C), the third hardest material in nature, has not been incorporated into many commercial applications because it exhibits anomalous failure when subjected to hypervelocity impact. To determine the atomistic origin of this brittle failure, we performed large-scale (~200000 atoms/cell) reactive-molecular-dynamics simulations of shear deformations of B4C, using the quantum-mechanics-derived reactive force field simulation. We examined the (0001)/(10l0) slip system related to deformation twinning and the (0111)/(1101) slip system related to amorphous band formation. We find that brittle failure in B4C arises from formation of higher density amorphous bands due to fracture of the icosahedra, a unique feature of these boron based materials. This leads to negative pressure and cavitation resulting in crack opening. Thus, to design ductile materials based on B4C we propose alloying aimed at promoting shear relaxation through intericosahedral slip that avoids icosahedral fracture. [ABSTRACT FROM AUTHOR]

The article reports on the de novo hierarchical simulation framework on high-end parallel supercomputers and clusters. The framework contains a high-end chemically reactive and non-reactive molecular dynamics simulations. It also includes an embedded divide-and-conquer algorithmic framework for linear-scaling simulation algorithms with minimal bandwidth complexity and tight error control. The framework also provides an adaptive hierarchical simulation with automated model transitioning aided by graph-based event tracking.

Preface to "Advances in Heterogeneous Catalysis and Electrocatalysis Including New Insights from Surface Science and Quantum Mechanics, Published in Honor of Professor Robert K. Grasselli, Irsee VIII Symposium Kloster Irsee, Germany 23-26 May 2019 (Irsee VIII)" The 8th Irsee Symposium entitled "Advances in Selective Oxidation Catalysis and Electrocatalysis Including New Insights from Surface Science and Quantum Mechanics", sponsored by the Robert Karl Grasselli Foundation, took place at the Schwäbisches Bildungszentrum in Irsee, Germany 23-26 May 2019. This was led off by Doug Buttrey with Reflections on the Life and Contributions of Robert Karl Grasselli, while Ferruccio Trifirò summarized common work with Bob on selective oxidation catalysis. [Extracted from the article]

We use our recent discovery of the reduction-coupled oxo activation (ROA) principle to design a series of organometallic molecules that activate C–H bonds through this unique proton/electron-decoupled hydrogen abstraction mechanism, in which the main group oxo moiety binds to the proton while the electron is transferred to the transition metal. Here we illustrate this general class of catalyst clusters with several examples that are validated through quantum mechanics calculations. [ABSTRACT FROM AUTHOR]

We recently developed the polarizable charge equilibration (PQEq) model to predict accurate electrostatic interactions for molecules and solids and optimized parameters for H, C, N, O, F, Si, P, S, and Cl elements to fit polarization energies computed by quantum mechanics (QM). Here, we validate and optimize the PQEq parameters for other p-block elements including Ge, As, Se, Br, Sn, Sb, Te, I, Pb, Bi, Po, and At using 28 molecular structures containing these elements. For these elements, we now include molecules with higher oxidation states: III and V for the As column, IV and VI for the Se column, and I, III, and V for the Br column. We find that PQEq predicts polarization energies in excellent agreement with QM. [ABSTRACT FROM AUTHOR]

Copper is the only elemental metal that reduces a significant fraction of CO2 to hydrocarbons and alcohols, but the atomistic reaction mechanism that controls the product distributions is not known because it has not been possible to detect the reaction intermediates on the electrode surface experimentally, or to carry out Quantum Mechanics (QM) calculations with a realistic description of the electrolyte (water). Here, we carry out QM calculations with an explicit description of water on the Cu(100) surface (experimentally shown to be stable under CO2 reduction reaction conditions) to examine the initial reaction pathways to form CO and formate (HCOO–) from CO2 through free energy calculations at 298 K and pH 7. We find that CO formation proceeds from physisorbed CO2 to chemisorbed CO2 (*CO2δ−), with a free energy barrier of ΔG⧧ = 0.43 eV, the rate-determining step (RDS). The subsequent barriers of protonating *CO2δ− to form COOH* and then dissociating COOH* to form *CO are 0.37 and 0.30 eV, respectively. HCOO– formation proceeds through a very different pathway in which physisorbed CO2 reacts directly with a surface H* (along with electron transfer), leading to ΔG⧧ = 0.80 eV. Thus, the competition between CO formation and HCOO– formation occurs in the first electron-transfer step. On Cu(100), the RDS for CO formation is lower, making CO the predominant product. Thus, to alter the product distribution, we need to control this first step of CO2 binding, which might involve controlling pH, alloying, or changing the structure at the nanoscale. [ABSTRACT FROM AUTHOR]

We develop here the methodology for dramatically accelerating the ReaxFF reactive force field based reactive molecular dynamics (RMD) simulations through use of the bond boost concept (BB), which we validate here for describing hydrogen combustion. The bond order, undercoordination, and overcoordination concepts of ReaxFF ensure that the BB correctly adapts to the instantaneous configurations in the reactive system to automatically identify the reactions appropriate to receive the bond boost. We refer to this as adaptive Accelerated ReaxFF Reactive Dynamics or aARRDyn. To validate the aARRDyn methodology, we determined the detailed sequence of reactions for hydrogen combustion with and without the BB. We validate that the kinetics and reaction mechanisms (that is the detailed sequences of reactive intermediates and their subsequent transformation to others) for H2 oxidation obtained from aARRDyn agrees well with the brute force reactive molecular dynamics (BF-RMD) at 2498 K. Using aARRDyn, we then extend our simulations to the whole range of combustion temperatures from ignition (798 K) to flame temperature (2998K), and demonstrate that, over this full temperature range, the reaction rates predicted by aARRDyn agree well with the BF-RMD values, extrapolated to lower temperatures. For the aARRDyn simulation at 798 K we find that the time period for half the H2 to form H2O product is ~538 s, whereas the computational cost was just 1289 ps, a speed increase of ~0.42 trillion (1012) over BF-RMD. In carrying out these RMD simulations we found that the ReaxFFCOH2008 version of the ReaxFF force field was not accurate for such intermediates as H30. Consequently we reoptimized the fit to a quantum mechanics (QM) level, leading to the ReaxFF—OH2014 force field that was used in the simulations. [ABSTRACT FROM AUTHOR]

We use quantum mechanics (QM) to determine the states formed by B dopants in diamond. We find that isolated B sites prefer to form BB dimers and that the dimers pair up to form tetramers (BBCBB) that prefer to aggregate parallel to the (111) surface in the <110> direction, one double layer below the H-terminated surface double layer. These tetramers lead to metallic character (Mott metal Insulator Transition) with holes in the valence band near the Γ point and electrons in the BBCBB tetramer promoted band along the X direction. Our experiments find very significant anisotropy in the superconductivity for boron-doped diamond thin films prepared with Microwave Plasma Assisted Chemical Vapor Deposition using deuterium-rich plasma. This leads to much higher conductivity in the X direction than the Y direction, as predicted by the QM. This phase transition to the anomalous phase is linked with the emergence of boson quantum entanglement states behaving as a bosonic insulating state. These anisotropic superconducting properties of the diamond film might enable applications such as single-photon detectors. We expect that this formation of a dirty superconductivity state is related to the BBCBB tetramers found in our QM calculations. [Display omitted] [ABSTRACT FROM AUTHOR]

Herein, we report a novel methodology for preparation of new N-doped extensively graphitized porous carbon (N-GPC) as a new catalyst support for Ru nanoparticles (NPs) with dramatically improved hydrogen evolution reaction (HER) activity. Our method is remarkably simple: pyrolyzing g-C 3 N 4 in the presence of Mg metal. Here, we show that Mg plays marvelous dual roles as a reducing agent to graphitize the g-C 3 N 4 precursor at low temperature and as a precursor for Mg 3 N 2 , which generates network-structured porous carbon as a new porogen. This offers highly robust graphitized carbon with high electrical conductivity, network-structured high porosity, and proper N content, most desired as a catalyst support. As-prepared Ru/N-GPC catalyst shows a remarkably low overpotential of 9.6 mV (vs. RHE) at 10 mA/cm2, which is near ideal, providing 12 times faster hydrogen production rate than state-of-the-art Pt/C. We explain the atomistic basis for this low overpotential and superb stability via Grand canonical quantum mechanics calculations. These calculations show that pyrrolic-N in the support strengthens the coupling to the Ru NP while weakening the binding of H to Ru NP to accelerate the Tafel step. [Display omitted] • N-doped extensively graphitized porous carbon (N-GPC) was prepared as a robust catalyst support for Ru nanoparticles (NPs) from calcination of g-C 3 N 4 with Mg. • The N-GPC shows high graphiticity and electrical conductivity, network-structured meso/macroporsity, and proper N content with rich pyrrolic-N species. • Ru-loaded N-GPC exhibits excellent HER activity with a remarkably low overpotential of 9.6 mV (vs. RHE) at 10 mA/cm2 and superb durability. • Grand canonical quantum mechanics calculations verify that pyrrolic-N in N-GPC strengthens the coupling to Ru NP while weakening the binding of H to Ru NP. [ABSTRACT FROM AUTHOR]

The Haber-Bosch industrial process for synthesis of ammonia (NH3) from hydrogen and nitrogen produces the millions of tons of ammonia gas annually needed to produce nitrates for fertilizers required to feed the earth's growing populations. This process has been optimized extensively, but it still uses enormous amounts of energy (2% of the world's supply), making it essential to dramatically improve its efficiency. To provide guidelines to accelerate this improvement, we used quantum mechanics to predict reaction mechanisms and kinetics for NH3 synthesis on Fe(111)--the best Fe single crystal surface for NH3 synthesis. We predicted the free energies of all reaction barriers for all steps in the mechanism and built these results into a kinetic Monte Carlo model for predicting steady state catalytic rates to compare with single-crystal experiments at 673 K and 20 atm. We find excellent agreement with a predicted turnover frequency (TOF) of 17.7 s-1 per 2 x 2 site (5.3 x 10-9 mol/cm²/sec) compared to TOΦ = 10 s-1 per site from experiment. [ABSTRACT FROM AUTHOR]

We report here the reaction mechanism for explicit aqueous solvent quantum mechanics (QM) studies determining the energetics and reaction barriers for the photocatalytic hydrogen evolution reaction (HER) on CH3NH3PbI3 surface. We find that both the lead (Pb) atoms and the surface organic molecules play essential roles, leading to a two-step Pb-activated amine-assisted (PbAAA) reaction mechanism involving an intermediate lead hydride state. Both H of H2 product are extracted from surface organic molecules, while two protons from the solution migrate along water chains via the Grotthuss mechanism to replace the H in organic molecule. We obtain a reaction barrier of 1.08 eV for photochemical generation of H2 on CH3NH3PbI3 compared to 2.61 eV for the dark reaction. We expect this HER mechanism can also apply to the other organic perovskites, but the energy barriers and reaction rates may depend on the basicity of electrolyte and intrinsic structures of perovskites. [ABSTRACT FROM AUTHOR]

The preparation and characterization of a series of isostructural cobalt complexes [Co(t-Bu)2PEPyEP(t-Bu)2(CH3CN)2][BF4]2 (Py = pyridine, E = CH2, NH, O, and X = BF4 (1a-c)) and the corresponding one-electron reduced analogues [Co(t-Bu)2PEPyEP(t-Bu)2(CH3CN)2][BF4]2 (2a-c) are reported. The reactivity of the reduced cobalt complexes with CO2, CO, and H+ to generate intermediates in a CO2 to CO and H2O reduction cycle are described. The reduction of 1a-c and subsequent reactivity with CO2 was investigated by cyclic voltammetry, and for 1a also by infrared spectroelectrochemistry. The corresponding CO complexes of (2a-c) were prepared, and the Co-CO bond strengths were characterized by IR spectroscopy. Quantum mechanical methods (B3LYP-d3 with solvation) were used to characterize the competitive reactivity of the reduced cobalt centers with H+ versus CO2. By investigating a series of isostructural complexes, correlations in reactivity with ligand electron withdrawing effects are made. [ABSTRACT FROM AUTHOR]

The polarizable charge equilibration (PQEq) method was developed to provide a simple but accurate description of the electrostatic interactions and polarization effects in materials. Previously, we optimized four parameters per element for the main group elements. Here, we extend this optimization to the 24 d-block transition-metal (TM) elements, columns 4–11 of the periodic table including Ti–Cu, Zr–Ag, and Hf–Au. We validate the PQEq description for these elements by comparing to interaction energies computed by quantum mechanics (QM). Because many materials applications involving TM are for oxides and other compounds that formally oxidize the metal, we consider a variety of oxidation states in 24 different molecular clusters. In each case, we compare interaction energies and induced fields from QM and PQEq along various directions. We find that the original χ and J parameters (electronegativity and hardness) related to the ionization of the atom remain valid; however, we find that the atomic radius parameter needs to be close to the experimental ionic radii of the transition metals. This leads to a much higher spring constant to describe the atomic polarizability. We find that these optimized parameters for PQEq provide accurate interaction energies compared to QM with charge distributions that depend in a reasonable way on the coordination number and oxidation states of the transition metals. We expect that this description of the electrostatic interactions for TM will be useful in molecular dynamics simulations of inorganic and organometallic materials. [ABSTRACT FROM AUTHOR]