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Electrochemical systems possess a considerable part of modern technologies, such as the operation of rechargeable batteries and the fabrication of electronic components, which are explored both experimentally and computationally. The largest gap between the experimental observations and atomic-level simulations is their orders-of-magnitude scale difference. While the largest computationally affordable scale of the atomic-level computations is ∼ ns and ∼ n m , the smallest reachable scale in the typical experiments, using very high-precision devices, is ∼ s and ∼ μ m. In order to close this gap and correlate the studies in the two scales, we establish an equivalent simulation setup for the given general experiment, which excludes the microstructure effects (i.e., solid–electrolyte interface), using the coarse-grained framework. The developed equivalent paradigm constitutes the adjusted values for the equivalent length scale (i.e., l E Q ), diffusivity (i.e., D E Q ), and voltage (i.e., V E Q ). The time scale for the formation and relaxation of the concentration gradients in the vicinity of the electrode matches for both smaller scale (i.e., atomistic) equivalent simulations and the larger scale (i.e., continuum) experiments and could be utilized for exploring the cluster-level inter-ionic events that occur during the extended time periods. The developed model could offer insights for forecasting experiment dynamics and estimating the transition period to the steady-state regime of operation. [ABSTRACT FROM AUTHOR]

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]

A highly active heterogenized molecular CO2 reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re+1(tBu-bpy)(CO)3Cl] (tBu-bpy = 4,4′-tert-butyl-2,2′-bipyridine) catalyst deposited on multiwalled carbon nanotubes are characterized using Re L3-edge x-ray absorption spectroscopy under electrochemical conditions and compared to the homogeneous catalyst. The Re oxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)3Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogeneous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using quantum mechanical calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts. [ABSTRACT FROM AUTHOR]

The non-uniform growth of microstructures in dendritic form inside the battery during prolonged charge–discharge cycles causes short-circuit as well as capacity fade. We develop a feedback control framework for the real-time minimization of such microstructures. Due to the accelerating nature of the branched evolution, we focus on the early stages of growth, identify the critical ramified peaks, and compute the effective time for the dissipation of ions from the vicinity of those branching fingers. The control parameter is a function of the maximum interface curvature (i.e., minimum radius) where the rate of runaway is the highest. The minimization of the total charging time is performed for generating the most packed microstructures, which correlate closely with those of considerably higher charging periods, consisting of constant and uniform square waves. The developed framework could be utilized as a smart charging protocol for safe and sustainable operation of rechargeable batteries, where the branching of the microstructures could be correlated with the sudden variation in the current/voltage. [ABSTRACT FROM AUTHOR]

We previously reported comprehensive density functional theory-molecular dynamics (DFT-MD) at 400 K to determine the composition and structure of the solid electrolyte interface (SEI) between a Li anode and [Pyr14][TFSI] ionic liquid. In this paper, we examined diffusion rates in both the Li-electrode region and SEI compact layer in smaller 83Li/2[TFSI] and larger 164Li/4[TFSI] systems. At 400 K, the Li-diffusion constant in the Li-region is 1.35 × 10−10 m2/s for 83Li/2[TFSI] and 5.64 × 10−10 m2/s for 164Li/4[TFSI], while for the SEI it is 0.33 × 10−10 m2/s and 0.22 × 10−10 m2/s, thus about one order slower in the SEI compared to the Li-region. This Li-diffusion is dominated by hopping from the neighbor shell of one F or O to the neighbor shell of another. Comparing the Li-diffusion at different temperatures, we find that the activation energy is 0.03 and 0.11 eV for the Li-region in the smaller and larger systems, respectively, while for the SEI it is 0.09 and 0.06 eV. [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]

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]

The densifying kinetics of lithium dendrites is characterized with effective activation energy of Ea ≈ 6 - 7 kcal mol-1 in our experiments and molecular dynamics computations. We show that heating lithium dendrites for 55 °C reduces the representative dendrites length λ(T,t) up to 36%. NVT reactive force field simulations on three-dimensional glass phase dendrites produced by our coarse grained Monte Carlo method reveal that for any given initial dendrite morphology, there is a unique stable atomic arrangement for a certain range of temperature, combined with rapid morphological transition (~10 ps) within quasi-stable states involving concurrent bulk and surface diffusions. Our results are useful for predicting the inherent structural characteristics of lithium dendrites such as dominant coordination number. [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]

Many important applications of electronic structure methods involve molecules or solid surfaces in a solvent medium. Since explicit treatment of the solvent in such methods is usually not practical, calculations often employ continuum solvation models to approximate the effect of the solvent. Previous solvation models either involve a parametrization based on atomic radii, which limits the class of applicable solutes, or based on solute electron density, which is more general but less accurate, especially for charged systems. We develop an accurate and general solvation model that includes a cavity that is a nonlocal functional of both solute electron density and potential, local dielectric response on this nonlocally determined cavity, and nonlocal approximations to the cavity-formation and dispersion energies. The dependence of the cavity on the solute potential enables an explicit treatment of the solvent charge asymmetry. With four parameters per solvent, this "CANDLE" model simultaneously reproduces solvation energies of large datasets of neutral molecules, cations, and anions with a mean absolute error of 1.8 kcal/mol in water and 3.0 kcal/mol in acetonitrile. [ABSTRACT FROM AUTHOR]

The laboratory performance of CIGS (Cu(In,Ga)Se2) based solar cells (20.8% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use density functional theory with the B3PW91 hybrid functional that we validate to provide very accurate descriptions of the band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the conduction band offset (CBO) of perfect CuInSe2/CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the valence band offset, and we find that Cd impurities do not significantly affect the CBO. Thus we show that Cu vacancies at the interface play the key role in enabling the tunability of CBO. We predict that Na further improves the CBO through electrostatically elevating the valence levels to decrease the CBO, explaining the observed essential role of Na for high performance. Moreover we find that K leads to a dramatic decrease in the CBO to 0.05 eV, much better than Na. We suggest that the efficiency of CIGS devices might be improved substantially by tuning the ratio of Na to K, with the improved phase stability of Na balancing phase instability from K. All these defects reduce interfacial stability slightly, but not significantly. [ABSTRACT FROM AUTHOR]

Highly excited heterogeneous complex materials are essential elements of important processes, ranging from inertial confinement fusion to semiconductor device fabrication. Understanding the dynamics of these systems has been challenging because of the difficulty in extracting mechanistic information from either experiment or theory. We describe here the electron force field (eFF) approximation to quantum mechanics which provides a practical approach to simulating the dynamics of such systems. eFF includes all the normal electrostatic interactions between electrons and nuclei and the normal quantum mechanical description of kinetic energy for the electrons, but contains two severe approximations: first, the individual electrons are represented as floating Gaussian wave packets whose position and size respond instantaneously to various forces during the dynamics; and second, these wave packets are combined into a many-body wave function as a Hartree product without explicit antisymmetrization. The Pauli principle is accounted for by adding an extra spin-dependent term to the Hamiltonian. These approximations are a logical extension of existing approaches to simulate the dynamics of fermions, which we review. In this paper, we discuss the details of the equations of motion and potentials that form eFF, and evaluate the ability of eFF to describe ground-state systems containing covalent, ionic, multicenter, and/or metallic bonds. We also summarize two eFF calculations previously reported on electronically excited systems: (1) the thermodynamics of hydrogen compressed up to ten times liquid density and heated up to 200 000 K; and (2) the dynamics of Auger fragmentation in a diamond nanoparticle, where hundreds of electron volts of excitation energy are dissipated over tens of femtoseconds. These cases represent the first steps toward using eFF to model highly excited electronic processes in complex materials. [ABSTRACT FROM AUTHOR]

We carried out quantum mechanical calculations (Perdew-Becke-Ernzerhof flavor of density functional theory) on 12.5% Y-doped BaZrO3 (BYZ) periodic structures to obtain energy barriers for intraoctahedral and interoctahedral proton transfers. We find activation energy (Ea) values of 0.48 and 0.49 eV for the intraoctahedral proton transfers on O–O edges (2.58 and 2.59 Å) of ZrO6 and YO6 octahedra, respectively, and Ea=0.41 eV for the interoctahedral proton transfer at O–O separation of 2.54 Å. These results indicate that both the interoctahedral and intraoctahedral proton transfers are important in the BYZ electrolyte. Indeed, the calculated values bracket the experimental value of Ea=0.44 eV. Based on the results obtained, the atomic level proton diffusion mechanism and possible proton diffusion pathways have been proposed for the BYZ electrolyte. The thermal librations of BO6 octahedra and uncorrelated thermal vibrations of the two oxygen atoms participating in the hydrogen bond lead to a somewhat chaotic fluctuation in the distances between the O atoms involved in the hydrogen bonding. Such fluctuations affect the barriers and at certain O–O distances allow the hydrogen atoms to move within the hydrogen bonds from one potential minimum to the other and between the hydrogen bonds. Concertation of these intra- and inter-H-bond motions results in continuous proton diffusion pathways. Continuity of proton diffusion pathways is an essential condition for fast proton transport. [ABSTRACT FROM AUTHOR]

We have parametrized a reactive force field for NaH, ReaxFFNaH, against a training set of ab initio derived data. To ascertain that ReaxFFNaH is properly parametrized, a comparison between ab initio heats of formation of small representative NaH clusters with ReaxFFNaH was done. The results and trend of ReaxFFNaH are found to be consistent with ab initio values. Further validation includes comparing the equations of state of condensed phases of Na and NaH as calculated from ab initio and ReaxFFNaH. There is a good match between the two results, showing that ReaxFFNaH is correctly parametrized by the ab initio training set. ReaxFFNaH has been used to study the dynamics of hydrogen desorption in NaH particles. We find that ReaxFFNaH properly describes the surface molecular hydrogen charge transfer during the abstraction process. Results on heat of desorption versus cluster size shows that there is a strong dependence on the heat of desorption on the particle size, which implies that nanostructuring enhances desorption process. To gain more insight into the structural transformations of NaH during thermal decomposition, we performed a heating run in a molecular dynamics simulation. These runs exhibit a series of drops in potential energy, associated with cluster fragmentation and desorption of molecular hydrogen. This is consistent with experimental evidence that NaH dissociates at its melting point into smaller fragments. [ABSTRACT FROM AUTHOR]

We present a new reactive force field ReaxFFHBN derived to accurately model large molecular and condensed phase systems of H, B, and N atoms. ReaxFFHBN has been tested against quantum calculation data for B–H, B–B, and B–N bond dissociations and for H–B–H, B–N–B, and N–B–N bond angle strain energies of various molecular clusters. The accuracy of the developed ReaxFFHBN for B–N–H systems is also tested for (i) H–B and H–B bond energies as a function of out of plane in H–B(NH2)3 and H–N(BH2)3, respectively, (ii) the reaction energy for the B3N3H6+H2→B3N3H8, and (iii) crystal properties such as lattice parameters and equations of states for the hexagonal type (h-BN) with a graphite structure and for the cubic type (c-BN) with a zinc-blende structure. For all these systems, ReaxFFHBN gives reliable results consistent with those from quantum calculations as it describes well bond breaking and formation in chemical processes and physical properties. Consequently, the molecular-dynamics simulation based on ReaxFFHBN is expected to give a good description of large systems (>2000 atoms even on the one-CPU machine) with hydrogen, boron, and nitrogen atoms. [ABSTRACT FROM AUTHOR]

Collision and adsorption of hydrogen with high incident kinetic energies on a single-walled boron nitride (BN) nanotube have been investigated. Molecular-dynamics (MD) simulations indicate that at incident energies below 14 eV hydrogen bounces off the BN nanotube wall. On the other hand, at incident energies between 14 and 22 eV each hydrogen molecule is dissociated at the exterior wall to form two hydrogen atoms, but only one of them goes through the wall. However, at the incident energies between 23 and 26 eV all of the hydrogen atoms dissociated at the exterior wall are found to be capable of going inside the nanotube and then to recombine to form hydrogen molecules inside the nanotube. Consequently, it is determined that hydrogen should have the incident energy >22 eV to go inside the nanotube. On the other hand, we find that the collisions using the incident energies >26 eV could result in damaging the nanotube structures. In addition our MD simulations find that hydrogen atoms dissociated at the wall cannot bind to either boron or nitrogen atoms in the interior wall of the nanotube. [ABSTRACT FROM AUTHOR]

The shear viscosity of liquid copper is studied using nonequilibrium molecular-dynamics simulations under planar shear flow conditions. We examined variation of viscosity as function of shear rate at a range of pressures (ca. 0 – 40 GPa). We analyzed these results using eight different phenomenological models and find that the observed non-Newtonian behavior is best described by the Powell–Eyring (PE) model: η(γ)=(η0-η∞)sinh-1(τγ)/(τγ)+η∞, where γ is the shear rate. Here η0 (the zero-shear-rate viscosity) extracted from the PE fit is in excellent agreement with available experimental data. The relaxation time τ from the PE fit describes the shear response to an applied stress. This provides the framework for interpreting the shear flow phenomena in complex systems, such as liquid metal and amorphous metal alloys. [ABSTRACT FROM AUTHOR]

The mechanism for the formation of hexamethylenetetraamine predicts the formation of aminomethanol from the addition of ammonia to formaldehyde. This molecule subsequently undergoes unimolecular decomposition to form methanimine and water. Aminomethanol is the predicted precursor to interstellar glycine, and is therefore of great interest for laboratory spectroscopic study, which would serve as the basis for observational searches. The height of the water loss barrier is therefore useful in the determination of an appropriate experimental approach for spectroscopic characterization of aminomethanol. We have determined the height of this barrier to be 55 kcal/mol at ambient temperatures. In addition, we have determined the infinite-pressure Rice–Ramsperger–Kassel–Marcus unimolecular decomposition rate to be <10-25 s-1 at 300 K, indicating gas-phase kinetic stability for typical laboratory and hot core temperatures. Therefore, spectroscopic characterization of and observational searches for this molecule should be straightforward provided an efficient formation mechanism can be found. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) has become the method of choice for many applications of quantum mechanics to the study of the electronic properties of molecules and solids. Despite the enormous progress in improving the functionals, the current generation is inadequate for many important applications. As part of the quest of finding better functionals, we consider in this paper the Perdew-Burke-Ernzerhof (PBE) functional, which we believe to have the best theoretical foundation, but which leads to unacceptable errors in predicting thermochemical data (heats of formation) of molecular systems [mean absolute deviation (MAD)=16.9 kcal/mol against the extended G2 data set of 148 molecules]. Much improved thermochemistry is obtained with hybrid DFT methods that include part of the Hartree-Fock exchange [thus B3LYP (Becke’s three parameter scheme combining Hartree-Fock exchange, Becke gradient corrected exchange functional and Lee-Yang-Parr correlational functional) with MAD=3.1 kcal/mol and PBE0 (Perdew’s hybrid scheme using PBE exchange and correlation functionals) with MAD=4.8 kcal/mol]. However we wish to continue the quest for a pure density-based DFT. Thus we optimized the four free parameters (μ, κ, α, and β) in PBE theory against experimental atomic data and the van der Waals interaction properties of Ne2, leading to the xPBE extended functional, which significantly outperforms PBE for thermochemical properties MAD reduced to 8.0 kcal/mol while being competitive or better than PBE for predictions of geometric parameters, ionization potentials, electron affinities, and proton affinities and for the description of van der Waals and hydrogen bond interactions. Thus xPBE significantly enlarges the field of applications available for pure DFT. The functional forms thus obtained for the exchange and correlational functionals may be useful for discovering new improved functionals or formalisms. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose a general approach for determining the entropy and free energy of complex systems as a function of temperature and pressure. In this method the Fourier transform of the velocity autocorrelation function, obtained from a short (20 ps) molecular dynamics trajectory is used to obtain the vibrational density of states (DoS) which is then used to calculate the thermodynamic properties by applying quantum statistics assuming each mode is a harmonic oscillator. This approach is quite accurate for solids, but leads to significant errors for liquids where the DoS at zero frequency, S(0), remains finite. We show that this problem can be resolved for liquids by using a two phase model consisting of a solid phase for which the DoS goes to zero smoothly at zero frequency, as in a Debye solid; and a gas phase (highly fluidic), described as a gas of hard spheres. The gas phase component has a DoS that decreases monotonically from S(0) and can be characterized with two parameters: S(0) and 3N[sup g], the total number of gas phase modes [3N[sup g]→0 for a solid and 3N[sup g]→3(N-1) for temperatures and pressures for which the system is a gas]. To validate this two phase model for the thermodynamics of liquids, we applied it to pure Lennard-Jones systems for a range of reduced temperatures from 0.9 to 1.8 and reduced densities from 0.05 to 1.10. These conditions cover the gas, liquid, crystal, metastable, and unstable states in the phase diagram. Our results compare quite well with accurate Monte Carlo calculations of the phase diagram for classical Lennard-Jones particles throughout the entire phase diagram. Thus the two-phase thermodynamics approach provides an efficient means for extracting thermodynamic properties of liquids (and gases and solids). © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We consider metallic alloys of Cu*, Cu, and Cu** in which the atoms differ only in their atomic radii and examine how the size ratio affects the local orders in the alloy systems. These studies use molecular dynamics simulations in which the atomic interactions are modeled with a Sutton–Chen many-body potential. Considering rapid cooling of these binary and ternary alloys from the melt, we find three regimes defined by the magnitude of atomic size ratio λ (λ≤1.0): with (i) large size ratios of 0.95<λ≤1.0, crystallization occurs; (ii) with moderate size ratios of 0.60≤λ≤0.95, a glass phase forms; and (iii) with small size ratios of λ<0.60, the alloy phase separates into pure phases and crystallize. From analyzing the structures of these binary and ternary alloys, we find that the liquid phase is characterized by local structures in which bonded atoms have local fivefold symmetry, which becomes more prominent as the glass phase forms. For phases that crystallize this local fivefold symmetry disappears as the long-range order of the crystalline phase dominates. The fivefold symmetry in the glass phase is mainly due to the icosahedral cluster formation. Energetically, the formation of icosahedral cluster is favored at the atomic size ratio of λ∼0.85, which is close to the λ at which our analyses shows the maximum in the fivefold symmetry and the number of icosahedral clusters. As λ decreases further, the phase separation is observed. The fivefold symmetry character and the number of icosahedral cluster shows the local minimum at this onset of the phase separation. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Methanediol, or methylene glycol, is a product of the liquid phase reaction of water and formaldehyde and is a predicted interstellar grain surface species. Detection of this molecule in a hot core environment would advance the understanding of complex organic chemistry in the interstellar medium, but its laboratory spectroscopic characterization is a prerequisite for such observational searches. This theoretical study investigates the unimolecular decomposition of methanediol, specifically the thermodynamic and kinetic stability of the molecule under typical laboratory and interstellar conditions. Methanediol was found to be thermodynamically stable at temperatures of <100 K, which is the characteristic temperature range for interstellar grain mantles. The infinite-pressure RRKM unimolecular decomposition rate was found to be <10[sup -18] s[sup -1] at 300 K, indicating gas phase kinetic stability for typical laboratory and hot core temperatures. Therefore, both laboratory studies of and observational searches for this molecule should be feasible. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the development of a highly efficient variant of the Monte Carlo method for direct evaluation of the partition function, free energy, and other configurational dependent physical properties for long polymer chains. Polymer model; Thermodynamic functions; Sampling; Efficiency.

Presents the Hindered Rotor Density-of-States (HRDS) interpolation function, which is asymptotically correct at low and high temperatures and accurate at intermediate temperatures. Low-temperature (quantum) limit; High-temperature (classical) limit; HRDS function; Thermodynamic functions; Applications.