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We present a novel formalism to characterize elastic heterogeneities in amorphous solids. In particular, we derive high-order strain-energy expansions for pairwise energies under athermal quasistatic dynamics. We then use the presented formalism to study the statistical properties of pairwise expansion coefficients and their link with the statistics of soft, quasilocalized modes, for a wide range of formation histories in both two- and three-dimensional systems. We further exploit the presented framework to access local yield stress maps by performing a non-linear stress–strain expansion within a cavity embedded in a frozen matrix. We show that our "bond micromechanics" compare well with the original "frozen matrix" method, with the caveat of overestimating large stress activations. We additionally show how local yield rules can be used as an input for a scalar elasto-plastic model (EPM) to predict the stress response of materials ranging from ductile to brittle. Finally, we highlight some of the limits of simple mesoscale models in capturing the aging dynamics of post-yielding systems. Intriguingly, we observe subdiffusive and diffusive shear band growths for particle-based simulations and EPMs, respectively. [ABSTRACT FROM AUTHOR]

Opinion dynamics is of paramount importance as it provides insights into the complex dynamics of opinion propagation and social relationship adjustment. It is assumed in most of the previous works that social relationships evolve much faster than opinions. This is not always true in reality. We propose an analytical approximation to study this issue for arbitrary time scales between opinion adjustment and network evolution. To this end, the coefficient of determination in statistics is introduced and a one-dimensional stable manifold is analytically found, i.e., the most likely trajectory. With the aid of the stable manifold, we further obtain the fate of opinions and the consensus time, i.e., fixation probability and fixation time. We find that for in-group bias, the more likely individuals are to adopt the popular opinion, the less likely the majority opinion takes over the population, i.e., conformity inhibits the domination of popular opinions. This counterintuitive result can be interpreted from a game perspective, in which in-group bias refers to a coordination game and rewiring probability refers to a rescaling of the selection intensity. Our work proposes an efficient approximation method to foster the understanding of opinion dynamics in dynamical networks. [ABSTRACT FROM AUTHOR]

We show that in quantum waveguides, the Fano resonance profile associated with propagating modes has its analogs with evanescent modes as well. This is found to be an unusual and a universal effect for any attractive potential. Further, we show that quantum dots or attractive impurity potentials embedded in the interior of a quantum waveguide yield significantly large Seebeck coefficient (thermopower) and power factor. Hence, they are good candidates for enhancing the thermoelectric energy conversion efficiency. We study the effect of a waveguide tapering on transport properties for the first time and the effect of curvature on the transmission coefficients. We propose a nanoscale current rectification device in two dimensions using tapered quantum waveguides. [ABSTRACT FROM AUTHOR]

Anisotropic fluid materials are of growing interest with the development of metamaterials and transformation acoustics. In the general three-dimensional case, such materials are characterized by a bulk modulus and a full symmetric matrix of density. Here, a method is presented to retrieve the bulk modulus and all six components of the density matrix from a selected set of six incident plane waves impinging on a layer of the material. From the six components of the density tensor, the three principal directions and the three principal densities of the material are recovered. The approach relies on the analytical expression of the reflection and transmission coefficients derived from a state vector analysis. It results in simple, closed-form, and easily-implementable inverse relations for the material parameters. As an illustration, the case of sound propagation through an orthorhombic lattice of overlapping air-filled ellipsoids is considered. The effective complex and frequency-dependent bulk modulus and density matrix are derived from homogenization cell problems and account for viscothermal losses. The retrieval method is then applied to the homogenized layer and results bear testament to its robustness to extract accurately all seven material parameters. This makes possible the characterization and design of anisotropic fluid materials in three dimensions. [ABSTRACT FROM AUTHOR]

Studies of power-modulated chlorine inductively coupled plasmas (ICPs) bounded by yttria-coated chamber walls are presented. Time-resolved optical emissions from Cl and Xe actinometry trace gas were recorded over the 740–920 nm region as power at 13.56 MHz was modulated between high power and no power. The intensity ratio of Cl-to-Xe emission, proportional to Cl number density, nCl, followed the modulation in power, allowing Cl heterogeneous loss coefficients, γCl, to be obtained from a simple time-resolved, 0-dimensional model of the afterglow period that best matched computed relative changes in nCl at the beginning and end of the powered period, with γCl as the only adjustable parameter. This approach only requires a treatment of diffusion and avoids complications introduced by attempting simulations of the full modulation period. Cl recombination coefficients were determined on the mostly yttria surfaces for Cl2 ICPs (a) immediately after NF3 plasma cleaning (γCl = 0.20), (b) during long exposure to the Cl2 plasma with no substrate bias (γCl = 0.11), and (c) during Si etching with substrate bias (γCl = 0.055-0.070). For Cl2/5% O2 ICPs, these values are 0.28, 0.17, and 0.030, respectively. These results compare favorably to qualitative behavior reported previously for continuous Cl2 and Cl2/O2 ICPs in this yttria-coated chamber. [ABSTRACT FROM AUTHOR]

We probe the electron attachment in hexafluoropropylene oxide (HFPO), C3F6O, a gas widely used in plasma technologies. We determine the absolute electron attachment cross section using two completely different experimental approaches: (i) a crossed-beam experiment at single collision conditions (local pressures of 5 × 10−4 mbar) and (ii) a pulsed Townsend experiment at pressures of 20–100 mbar. In the latter method, the cross sections are unfolded from the electron attachment rate coefficients. The cross sections derived independently by the two methods are in very good agreement. We additionally discuss the dissociative electron attachment fragmentation patterns and their role in the radical production in industrial HFPO plasmas. [ABSTRACT FROM AUTHOR]

We describe a novel approach to quantifying the uncertainty inherent in the chemical kinetic master equation with stochastic coefficients. A stochastic collocation method is coupled to an analytical expansion of the master equation to analyze the effects of both extrinsic and intrinsic noise. The method consists of an analytical moment-closure method resulting in a large set of differential equations with stochastic coefficients that are in turn solved via a Smolyak sparse grid collocation method. We discuss the error of the method relative to the dimension of the model and clarify which methods are most suitable for the problem. We apply the method to two typical problems arising in chemical kinetics with time-independent extrinsic noise. Additionally, we showagreement with classical Monte Carlo simulations and calculate the variance over time as the sum of two expectations. The method presented here has better convergence properties for low to moderate dimensions than standard Monte Carlo methods and is therefore a superior alternative in this regime. [ABSTRACT FROM AUTHOR]

The Uhlenbeck-Ford (UF) model was originally proposed for the theoretical study of imperfect gases, given that all its virial coefficients can be evaluated exactly, in principle. Here, in addition to computing the previously unknown coefficients B11 through B13, we assess its applicability as a reference system in fluid-phase free-energy calculations using molecular simulation techniques. Our results demonstrate that, although the UF model itself is too soft, appropriately scaled Uhlenbeck-Ford (sUF) models provide robust reference systems that allow accurate fluid-phase free-energy calculations without the need for an intermediate reference model. Indeed, in addition to the accuracy with which their free energies are known and their convenient scaling properties, the fluid is the only thermodynamically stable phase for a wide range of sUF models. This set of favorable properties may potentially put the sUF fluid-phase reference systems on par with the standard role that harmonic and Einstein solids play as reference systems for solid-phase free-energy calculations. [ABSTRACT FROM AUTHOR]

The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (~0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger. [ABSTRACT FROM AUTHOR]

We provide a full set of growth rate coefficients to enable high-accuracy two- and three-dimensional simulations of dry thermal oxidation of 4H-silicon carbide. The available models are insufficient for the simulation of complex multi-dimensional structures, as they are unable to predict oxidation for arbitrary crystal directions because of the insufficient growth rate coefficients. By investigating time-dependent dry thermal oxidation kinetics, we obtain temperature-dependent growth rate coefficients for surfaces with different crystal orientations. We fit experimental data using an empirical relation to obtain the oxidation growth rate parameters. Time-dependent oxide thicknesses at various temperatures are taken from published experimental findings. We discuss the oxidation rate parameters in terms of surface orientation and oxidation temperature. Additionally, we fit the obtained temperature-dependent growth rate coefficients using the Arrhenius equation to obtain activation energies and pre-exponential factors for the four crystal orientations. The thereby obtained parameters are essential for enabling high-accuracy simulations of dry thermal oxidation and can be directly used to augment multi-dimensional process simulations. [ABSTRACT FROM AUTHOR]

We propose a method for calculating the non-radiative decay rates for polyatomic molecules including anharmonic effects of the potential energy surface (PES) in the Franck-Condon region. The method combines the n-mode representation method to construct the ab initio PES and the nearly exact time-dependent density matrix renormalization group method (TD-DMRG) to simulate quantum dynamics. In addition, in the framework of TD-DMRG, we further develop an algorithm to calculate the final-state-resolved rate coefficient which is very useful to analyze the contribution from each vibrational mode to the transition process. We use this method to study the internal conversion (IC) process of azulene after taking into account the anharmonicity of the ground state PES. The results show that even for this semi-rigid molecule, the intramode anharmonicity enhances the IC rate significantly, and after considering the two-mode coupling effect, the rate increases even further. The reason is that the anharmonicity enables the C–H vibrations to receive electronic energy while C–H vibrations do not contribute on the harmonic PES as the Huang-Rhys factor is close to 0. [ABSTRACT FROM AUTHOR]

In order to reduce the operation count in vibration correlation methods, e.g., vibrational configuration interaction (VCI) theory, a tensor decomposition approach has been applied to the analytical representations of multidimensional potential energy surfaces (PESs). It is shown that a decomposition of the coefficients within the individual n-mode coupling terms in a multimode expansion of the PES is feasible and allows for convenient contractions of one-dimensional integrals with these newly determined factor matrices. Deviations in the final VCI frequencies of a set of small molecules were found to be negligible once the rank of the factors matrices is chosen appropriately. Recommendations for meaningful ranks are provided and different algorithms are discussed. [ABSTRACT FROM AUTHOR]

In this work, we investigate how atomic C6 coefficients and static dipole polarizabilities α scale with effective volume. We show, using confined atoms covering rows 1-5 of the periodic table, that C6/C6R ≈ (V/V R)pZ and α/αR ≈ (V/V R)p' Z (for volume V = ƒ dr 4π/3 r³n(r)), where C6R, αR, and VR are the reference values and effective volume of the free atom. The scaling exponents pZ and p' Z vary substantially as a function of element number Z = N, in contrast to the standard "rule of thumb" that pZ = 2 and p'Z = 1. Remarkably, we find that the polarizability and C6 exponents p' and p are related by p' ≈ p - 0.615 rather than the expected p' ≈ p/2. Results are largely independent of the form of the confining potential (harmonic, cubic, and quartic potentials are considered) and kernel approximation, justifying this analysis. [ABSTRACT FROM AUTHOR]

The response of dielectric media to electromagnetic fields can be described by using either the response to a Maxwell field E or to an externally produced field E. The former response is measured by phenomenological (dielectric) coefficients and the latter by susceptibilities. With the purpose of clarifying some recent proposals, the connections between the linear (two-point) and first non-vanishing nonlinear (four-point) dielectric coefficients and the susceptibilities for media confined to a slab are examined using a general procedure developed sometime ago. Unlike the relations found for correlations between a local polarization density and the integrated polarization densities (total polarizations), the point-point connections give rise to non-vanishing cross correlations between polarization densities which are parallel and perpendicular to the slab surfaces. The cross correlations in the two-point connections vanish when one polarization density is integrated to form the correlations between a local polarization density and the total polarization thereby losing angular information. The integrated parallel and perpendicular correlations remain different. When the fourpoint connections are similarly integrated most, but not all, cross correlations vanish. The angular correlations induced by the slab surfaces render the use of point-point correlations that are valid for isotropic media invalid for use in the integrated slab densities. In addition, the nonlinear fluctuations in the perpendicular components are drastically reduced relative to those in the parallel components or in isotropic media. [ABSTRACT FROM AUTHOR]

Mutual neutralization (MN) rate coefficients kMN for He+ with the anions Cl-, Br-, I-, and SF6 - are reported from 300 to 500 K. The measured rate coefficients may contain a contribution from transfer ionization, i.e., double ionization of the anion. The large rate coefficient for He+ + SF6 - (2.4 × 10-7 cm³ s-1 at 300 K) is consistent with earlier polyatomic MN results found to have a reduced mass dependence of µ-1/2. Neutralization of He+ by the atomic halides follows the trend observed earlier for Ne+, Ar+, Kr+, and Xe+ neutralized by atomic halides, kMN (Cl-) < kMN (Br-) < kMN (I-). Only an upper limit could be measured for the neutralization of He+ by Cl-. Predictions of the rate coefficients from a previously proposed simple model of atomic-atomic MN results are consistent with the present He+-halide rate coefficients. The temperature dependences are modestly negative for Br- and I-, while that for SF6- is small or negligible. [ABSTRACT FROM AUTHOR]

Mutual neutralization (MN) rate coefficients kMN for He+ with the anions Cl-, Br-, I-, and SF6 - are reported from 300 to 500 K. The measured rate coefficients may contain a contribution from transfer ionization, i.e., double ionization of the anion. The large rate coefficient for He+ + SF6 - (2.4 × 10-7 cm³ s-1 at 300 K) is consistent with earlier polyatomic MN results found to have a reduced mass dependence of µ-1/2. Neutralization of He+ by the atomic halides follows the trend observed earlier for Ne+, Ar+, Kr+, and Xe+ neutralized by atomic halides, kMN (Cl-) < kMN (Br-) < kMN (I-). Only an upper limit could be measured for the neutralization of He+ by Cl-. Predictions of the rate coefficients from a previously proposed simple model of atomic-atomic MN results are consistent with the present He+-halide rate coefficients. The temperature dependences are modestly negative for Br- and I-, while that for SF6- is small or negligible. [ABSTRACT FROM AUTHOR]

Results of a systematic comparison of the MoxOy- + H2O and WxOy - + H2O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. WxOy- clusters undergo more direct oxidation by water to yield WxOy+1- + H2, while for MoxOy- clusters, production of MoxOyH2- (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, MoxOy- clusters generally have higher rate coefficients than analogous WxOy- clusters if MoxOy+1H2- formation is included. Results of calculations on the M2Oy- + H2O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to MxOy+1H2- formation. The results predict that the Lewis acid-base complex is more strongly bound for MoxOy- clusters than for WxOy- clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most MoxOy- + H2O reactions, and the WxOy- + H2O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H2 production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures. [ABSTRACT FROM AUTHOR]

Ginzburg-Landau-Devonshire models are used to explore ferroelectric phases and pyroelectric coefficients of symmetric free-standing, thin-film trilayer heterostructures composed of a ferroelectric and two identical non-ferroelectric layers. Using BaTiO3 as a model ferroelectric, we explore the influence of temperature, in-plane misfit strain, and the non-ferroelectric layer (including effects of elastic compliance and volume fraction) on the phase evolution in the ferroelectric. The resulting phase diagram reveals six stable phases, two of which are not observed for thin films on semi-infinite cubic substrates. From there, we focus on heterostructures with non-ferroelectric layers of commonly available scandate materials which are widely used as substrates for epitaxial growth. Again, six phases with volatile phase boundaries are found in the phase diagram for the NdScO3/BaTiO3/NdScO3 trilayerheterostructures. The evolution of polarization and pyroelectric coefficients in the free-standing NdScO3 trilayer heterostructures is discussed with particular attention to the role that heterostructure design plays in influencing the phase evolution and temperature-dependence with a goal of creating enhanced pyroelectric response and advantages over traditional thin-film heterostructures. [ABSTRACT FROM AUTHOR]

Recent single molecule measurements of biomolecular folding achieved the time resolution sufficient for observation of individual transition paths. This note discusses several ways in which transition path ensembles can be statistically analyzed to extract a single, "typical" transition path. Analytical approximations derived here for such a transition path further allow one to estimate dynamical parameters (such as the intramolecular diffusion coefficient) directly from the transition path shapes. [ABSTRACT FROM AUTHOR]

The observed abundances of the methylidyne cation, CH+, in diffuse molecular clouds can be two orders of magnitude higher than the prediction of the standard gas-phase models which, in turn, predict rather well the abundances of neutral CH. It is therefore necessary to investigate all the possible formation and destruction processes of CH+ in the interstellar medium with the most abundant species H, H2, and e-. In this work, we address the destruction process of CH+ by hydrogen abstraction. We report a new calculation of the low temperature rate coefficients for the abstraction reaction, using accurate time-independent quantum scattering and a new high-level ab initio global potential energy surface including a realistic model of the long-range interaction between the reactants H and CH+. The calculated thermal rate coefficient is in good agreement with the experimental data in the range 50 K-800 K. However, at lower temperatures, the experimental rate coefficient takes exceedingly small values which are not reproduced by the calculated rate coefficient. Instead, the latter rate coefficient is close to the one given by the Langevin capture model, as expected for a reaction involving an ion and a neutral species. Several recent theoretical works have reported a seemingly good agreement with the experiment below 50 K, but an analysis of these works show that they are based on potential energy surfaces with incorrect long-range behavior. The experimental results were explained by a loss of reactivity of the lowest rotational states of the reactant; however, the quantum scattering calculations show the opposite, namely, a reactivity enhancement with rotational excitation. [ABSTRACT FROM AUTHOR]

Detailed measurements employing a combination of a cryogenic flowing afterglow with Langmuir probe (Cryo-FALP II) and a stationary afterglow with near-infrared absorption spectroscopy (SACRDS) show that binary electron recombination of para-H+3 and ortho-H+3 ions occurs with significantly different rate coefficients, pabin and oabin, especially at very lowtemperatures. The measurements cover temperatures from 60Kto 300 K. At the lowest temperature of 60 K, recombination of para-H+3 is at least three times faster than that of ortho-H+3 (pabin = (1.8 ± 0.4) × 10-7 cm³ s-1 vs. oabin = (0+5-0) × 10-8 cm³ s-1). [ABSTRACT FROM AUTHOR]

A new algorithm to solve the Time Dependent Density Functional Theory (TDDFT) equations in the space of the density fitting auxiliary basis set has been developed and implemented. The method extracts the spectrum from the imaginary part of the polarizability at any given photon energy, avoiding the bottleneck of Davidson diagonalization. The original idea which made the present scheme very efficient consists in the simplification of the double sum over occupied-virtual pairs in the definition of the dielectric susceptibility, allowing an easy calculation of such matrix as a linear combination of constant matrices with photon energy dependent coefficients. The method has been applied to very different systems in nature and size (from H2 to [Au147]-). In all cases, the maximum deviations found for the excitation energies with respect to the Amsterdam density functional code are below 0.2 eV. The new algorithm has the merit not only to calculate the spectrum at whichever photon energy but also to allow a deep analysis of the results, in terms of transition contribution maps, Jacob plasmon scaling factor, and induced density analysis, which have been all implemented. [ABSTRACT FROM AUTHOR]

In this paper, we define and experimentally verify thermodynamic characteristics of the liquid-glass transition, taking into account a kinetic origin of the process. Using the density scaling law and the four-point measure of the dynamic heterogeneity of molecular dynamics of glass forming liquids, we investigate contributions of enthalpy, temperature, and density fluctuations to spatially heterogeneous molecular dynamics at the liquid-glass transition, finding an equation for the pressure coefficient of the glass transition temperature, dTg/dp. This equation combined with our previous formula for dTg/dp, derived solely from the density scaling criterion, implies a relationship among thermodynamic coefficients at Tg. Since this relationship and both the equations for dTg/dp are very well validated using experimental data at Tg, they are promising alternatives to the classical Prigogine-Defay ratio and both the Ehrenfest equations in case of the liquid-glass transition. [ABSTRACT FROM AUTHOR]

The temperature-dependent diffusion coefficient of interstitial helium in zirconium carbide (ZrC) matrix is calculated based on the transition state theory. The microscopic parameters in the activation energy and prefactor are obtained from first-principles total energy and phonon frequency calculations including the all atoms. The obtained activation energy is 0.78 eV, consistent with experimental value. Besides, we evaluated the influence of C and Zr vacancies as the perturbation on helium diffusion, and found the C vacancy seems to confine the mobility of helium and the Zr vacancy promotes helium diffusion in some extent. These results provide a good reference to understand the behavior of helium in ZrC matrix. [ABSTRACT FROM AUTHOR]

In this paper, we present axisymmetric numerical simulations of shock propagation in air over an aluminum particle for Mach numbers up to 10. The numerical method is a finite-volume based solver on a Cartesian grid that allows for multi-material interfaces and shocks. Validation of the solver is demonstrated by comparing to existing experimental data. We compute the unsteady inviscid drag coefficient as a function of time, and show that when normalized by post-shock conditions, the maximum drag coefficient decreases with Mach number. Furthermore, for supercritical Mach numbers, we show that the inviscid steady-state drag asymptotes to a non-zero value due to the presence of a bow shock formed just upstream of the particle. Using this information, we also present a simplified point-particle force model that can be used for mesoscale simulations. Finally, we investigate the dynamics of a shock propagating over a 1-D array of particles aligned in the flow direction. We show that the maximum drag coefficient increases as the shock travels deep into the array and then asymptotes to a final value, which can be as high as 50% more than that of the first particle, depending on Mach number and particle spacing. [ABSTRACT FROM AUTHOR]

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021cm-3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted. [ABSTRACT FROM AUTHOR]

The piezoelectric property of [001]-oriented 0.5%MnO2-(K0.5Na0.5)NbO3 (Mn-KNN) crystals was studied as a function of domain size, being poled with different electric fields at 205 °C (above orthorhombic to tetragonal phase transition temperature To-t). The piezoelectric coefficients d33 and relative dielectric constants er were found to increase from 270 pC/N to 350 pC/N and 730 to 850 with the domain size decreasing from 9 to 2 lm, respectively. The thermal stability of piezoelectric property was investigated, where the d33 value for [001]-oriented Mn-KNN crystals with domain size of 2 lm was found to decrease to 330 pC/N at depoling temperature of 150 °C, with minimal variation of ∼6%. The results reveal that domain size engineering is an effective way to improve the piezoelectric properties of Mn-KNN crystals. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) coefficients can be accurately generated and understood by modelling the dynamic multipole polarizability of each interacting object. Accurate static polarizabilities are the key to accurate dynamic polarizabilities and vdW coefficients. In this work, we present and study in detail a hollow-sphere model for the dynamic multipole polarizability proposed recently by two of the present authors (JT and JPP) to simulate the vdW coefficients for inhomogeneous systems that allow for a cavity. The inputs to this model are the accurate static multipole polarizabilities and the electron density. A simplification of the full hollow-sphere model, the single-frequency approximation (SFA), circumvents the need for a detailed electron density and for a double numerical integration over space. We find that the hollow-sphere model in SFA is not only accurate for nanoclusters and cage molecules (e.g., fullerenes) but also yields vdW coefficients among atoms, fullerenes, and small clusters in good agreement with expensive time-dependent density functional calculations. However, the classical shell model (CSM), which inputs the static dipole polarizabilities and estimates the static higher-order multipole polarizabilities therefrom, is accurate for the higher-order vdW coefficients only when the interacting objects are large. For the lowest-order vdW coefficient C6, SFA and CSM are exactly the same. The higher-order (C8 and C10) terms of the vdW expansion can be almost as important as the C6 term in molecular crystals. Application to a variety of clusters shows that there is strong non-additivity of the long-range vdW interactions between nanoclusters. [ABSTRACT FROM AUTHOR]

Density functional theory based ab initio molecular dynamics is used to obtain microscopic details of the interactions in sodium chloride solutions. By following the changes in the atomic C6 coefficients under the Tkatchenko-Scheffler's scheme, we were able to identify two different coordination situations for the Cl- ion with significant different capabilities to perform dispersion interactions. This capability is enhanced when the ion-ion distance corresponds to the contact ion-pair situation. Also, the oxygen and hydrogen atoms of the water molecules change their aptitudes to interact through van der Waals like terms when they are close to the cation region of the ion-pair. These results have interesting implications on the design of force fields to model electrolyte solutions. [ABSTRACT FROM AUTHOR]

Given the ubiquity of depletion effects in biological and other soft matter systems, it is desirable to have coarse-grained Molecular Dynamics (MD) simulation approaches appropriate for the study of complex systems. This paper examines the use of two common truncated Lennard-Jones (Weeks-Chandler-Andersen (WCA)) potentials to describe a pair of colloidal particles in a thermal bath of depletants. The shifted-WCA model is the steeper of the two repulsive potentials considered, while the combinatorial-WCA model is the softer. It is found that the depletion-induced well depth for the combinatorial-WCA model is significantly deeper than the shifted-WCA model because the resulting overlap of the colloids yields extra accessible volume for depletants. For both shifted- and combinatorial-WCA simulations, the second virial coefficients and pair potentials between colloids are demonstrated to be well approximated by the Morphometric Thermodynamics (MT) model. This agreement suggests that the presence of depletants can be accurately modelled in MD simulations by implicitly including them through simple, analytical MT forms for depletion-induced interactions. Although both WCA potentials are found to be effective generic coarse-grained simulation approaches for studying depletion effects in complicated soft matter systems, combinatorial-WCA is the more efficient approach as depletion effects are enhanced at lower depletant densities. The findings indicate that for soft matter systems that are better modelled by potentials with some compressibility, predictions from hard-sphere systems could greatly underestimate the magnitude of depletion effects at a given depletant density. [ABSTRACT FROM AUTHOR]

In this paper, it is shown that the numerical differentiation method in performing the coupling parameter series expansion [S. Zhou, J. Chem. Phys. 125, 144518 (2006); AIP Adv. 1, 040703 (2011)] excels at calculating the coefficients a of hard sphere high temperature series expansion (HS-HTSE) of the free energy. Both canonical ensemble and isothermal-isobaric ensemble Monte Carlo simulations for fluid interacting through a hard sphere attractive Yukawa (HSAY) potential with extremely short ranges and at very low temperatures are performed, and the resulting two sets of data of thermodynamic properties are in excellent agreement with each other, and well qualified to be used for assessing convergence of the HS-HTSE for the HSAY fluid. Results of valuation are that (i) by referring to the results of a hard sphere square well fluid [S. Zhou, J. Chem. Phys. 139, 124111 (2013)], it is found that existence of partial sum limit of the high temperature series expansion series and consistency between the limit value and the true solution depend on both the potential shapes and temperatures considered. (ii) For the extremely short range HSAY potential, the HS-HTSE coefficients a falls rapidly with the order i, and the HS-HTSE converges from fourth order; however, it does not converge exactly to the true solution at reduced temperatures lower than 0.5, wherein difference between the partial sum limit of the HS-HTSE series and the simulation result tends to become more evident. Something worth mentioning is that before the convergence order is reached, the preceding truncation is always improved by the succeeding one, and the fourth- and higher-order truncations give the most dependable and qualitatively always correct thermodynamic results for the HSAY fluid even at low reduced temperatures to 0.25. [ABSTRACT FROM AUTHOR]

The second-order vibrational Hamiltonian of a semi-rigid polyatomic molecule when resonances are present can be reduced to a quasi-diagonal form using second-order vibrational perturbation theory. Obtaining exact vibrational energy levels requires subsequent numerical diagonalization of the Hamiltonian matrix including the first- and second-order resonance coupling coefficients. While the first-order Fermi resonance constants can be easily calculated, the evaluation of the secondorder Darling-Dennison constants requires more complicated algebra for seven individual cases with different numbers of creation-annihilation vibrational quanta. The difficulty in precise evaluation of the Darling-Dennison coefficients is associated with the previously unrecognized interference with simultaneously present Fermi resonances that affect the form of the canonically transformed Hamiltonian. For the first time, we have presented the correct form of the general expression for the evaluation of the Darling-Dennison constants that accounts for the underlying effect of Fermi resonances. The physically meaningful criteria for selecting both Fermi and Darling-Dennison resonances are discussed and illustrated using numerical examples. [ABSTRACT FROM AUTHOR]

An explicit formulation of the rotational relaxation time in terms of state-to-state rate coefficients associated to inelastic collisions is reported. The state-to-state rates needed for the detailed interpretation of relaxation in H2 and D2, including isotopic variant mixtures, have been calculated by solving the close-coupling Schrödinger equations using the H2–H2 potential energy surface by Diep and Johnson [J. Chem. Phys.112, 4465 (2000)]. Relaxation related quantities (rotational effective cross section, bulk viscosity, relaxation time, and collision number) calculated from first principles agree reasonably well with acoustic absorption experimental data on H2 and D2 between 30 and 293 K. This result confirms at once the proposed formulation, and the validation of the H2–H2 potential energy surface employed, since no approximations have been introduced in the dynamics. Accordingly, the state-to-state rates derived from Diep and Johnson potential energy surface appear to be overestimated by up to 10% for H2, and up to 30% for D2 at T = 300 K, showing a better agreement at lower temperatures. [ABSTRACT FROM AUTHOR]

In this work, we present a method, called the DQ scheme (where D and Q stand for dipole and quadrupole, respectively), for transforming a set of adiabatic electronic states to diabatic states by using the dipole and quadrupole moments to determine the transformation coefficients. It is more broadly applicable than methods based only on the dipole moment; for example, it is not restricted to electron transfer reactions, and it works with any electronic structure method and for molecules with and without symmetry, and it is convenient in not requiring orbital transformations. We illustrate this method by prototype applications to two cases, LiH and phenol, for which we compare the results to those obtained by the fourfold-way diabatization scheme [ABSTRACT FROM AUTHOR]

The influence of physical aging on the ß relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the ß relaxation (ΔG") decreases while its relaxation time ( τß) increases during aging. We find that, as in organic glasses, the changes of ln(τß) and ln(τGmax) are linearly correlated with ln(τ=b-a ln(G"max). This behavior is discussed in term of the asymmetric double-well potential (ADWP) model, with U and Δ the energies characterizing the ADWP. It is suggested that during aging the ratio U / Δ remains approximately constant, with a value close to the coefficient describing the linear correlation between ln(τß) and ln(G"max)(U/Δ ~ a). Moreover, the evolution versus aging time of ΔGmax can be described by a simple stretched exponential equation giving values of Taging consistent with tan(5) measurements during aging. The very similar behavior of the ß relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation. [ABSTRACT FROM AUTHOR]

The thermodynamics of iron clusters of various sizes, from 76 to 2452 atoms, typical of the catalyst particles used for carbon nanotubes growth, has been explored by a flat histogram Monte Carlo (MC) algorithm (called the a -mapping), developed by Soudan et al. [J. Chem. Phys. 135, 144109 (2011), Paper I]. This method provides the classical density of states, gp(Ep) in the configurational space, in terms of the potential energy of the system, with good and well controlled convergence properties, particularly in the melting phase transition zone which is of interest in this work. To describe the system, an iron potential has been implemented, called "corrected EAM" (cEAM), which approximates the MEAM potential of Lee et al. [Phys. Rev. B 64, 184102 (2001)] with an accuracy better than 3 meV/at, and a five times larger computational speed. The main simplification concerns the angular dependence of the potential, with a small impact on accuracy, while the screening coefficients Sij are exactly computed with a fast algorithm. With this potential, ergodic explorations of the clusters can be performed efficiently in a reasonable computing time, at least in the upper half of the solid zone and above. Problems of ergodicity exist in the lower half of the solid zone but routes to overcome them are discussed. The solid-liquid (melting) phase transition temperature Tm is plotted in terms of -- 1/3 the cluster atom number Nat. The standard Nat -1/3 linear dependence (Pawlow law) is observed for Nat >300, allowing an extrapolation up to the bulk metal at 1940 ±50 K. For Nat <150, a strong divergence is observed compared to the Pawlow law. The melting transition, which begins at the surface, is stated by a Lindemann-Berry index and an atomic density analysis. Several new features are obtained for the thermodynamics of cEAM clusters, compared to the Rydberg pair potential clusters studied in Paper I. [ABSTRACT FROM AUTHOR]

Rate coefficients can fluctuate in statically and dynamically disordered kinetics. Here, we relate the rate coefficient for an irreversibly decaying population to the Fisher information. From this relationship we define kinetic versions of statistical-length squared and divergence that measure cumulative fluctuations in the rate coefficient. We show the difference between these kinetic quantities measures the amount of disorder, and is zero when the rate coefficient is temporally and spatially unique. [ABSTRACT FROM AUTHOR]

The regularity of the existing data on the virial coefficients for the Lennard-Jones and modified Lennard-Jones models has allowed a rough extrapolation to the coefficients of higher orders. The corresponding approximation of the infinite virial series has been proposed for the limited temperature interval: 0.4-0.8 of the critical temperature. The loci of zero points of isothermal bulk modulus obtained on the basis of this approximation are close to the vapor-liquid branch of the experimental binodal rather than spinodal. In addition, those points ((dP/dV)T = 0) almost coincide with the divergence points of the approximated virial series that may eliminate some disputable questions about the boundaries of adequacy for the virial equation of state and makes the theoretical isotherms qualitatively similar to the real in the condensation region. [ABSTRACT FROM AUTHOR]

We investigate the thermal expansion behaviors of monolayer graphene and three configurations of graphene with point defects, namely the replacement of one carbon atom with a boron or nitrogen atom, or of two neighboring carbon atoms by boron-nitrogen atoms, based on calculations using first-principles density functional theory. It is found that the thermal contraction of monolayer graphene is significantly decreased by point defects. Moreover, the corresponding temperature for negative linear thermal expansion coefficient with the maximum absolute value is reduced. The cause is determined to be point defects that enhance the mechanical strength of graphene and then reduce the amplitude and phonon frequency of the out-of-plane acoustic vibration mode. Such defect weakening of graphene thermal contraction will be useful in nanotechnology to diminish the mismatching or strain between the graphene and its substrate. [ABSTRACT FROM AUTHOR]

The basic formulation for the multifacet generalization of the graphically contracted function (MFGCF) electronic structure method is presented. The analysis includes the discussion of linear dependency and redundancy of the arc factor parameters, the computation of reduced density matrices, Hamiltonian matrix construction, spin-density matrix construction, the computation of optimization gradients for single-state and state-averaged calculations, graphical wave function analysis, and the efficient computation of configuration state function and Slater determinant expansion coefficients. Timings are given for Hamiltonian matrix element and analytic optimization gradient computations for a range of model problems for full-CI Shavitt graphs, and it is observed that both the energy and the gradient computation scale as O(N²n4) for N electrons and n orbitals. The important arithmetic operations are within dense matrix-matrix product computational kernels, resulting in a computationally efficient procedure. An initial implementation of the method is used to present applications to several challenging chemical systems, including N2 dissociation, cubic H8 dissociation, the symmetric dissociation of H2O, and the insertion of Be into H2. The results are compared to the exact full-CI values and also to those of the previous single-facet GCF expansion form. [ABSTRACT FROM AUTHOR]

The recent extension of Rosenfeld's fundamental measure theory to anisotropic hard particles predicts nematic order of rod-like particles. Our analytic study of different aligned shapes provides new insights into the structure of this density functional, which is basically founded on experience with hard spheres. We combine scaling arguments with dimensional crossover and motivate a modified expression, which enables an appropriate description of smectic layering. We calculate the nematicsmectic- A transition of monodisperse hard spherocylinders with and without orientational degrees of freedom and present the equation of state and phase diagram including these two liquid crystalline phases in good agreement with simulations. We also find improved results related to the isotropic-nematic interface. We discuss the quality of empirical corrections and the convergence towards an exact second virial coefficient, including higher order terms. [ABSTRACT FROM AUTHOR]

We use charge transient spectroscopy to study the trap levels in the prototypical polymeric semiconductor poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Emission signature of the traps has been resolved using higher order spectroscopic analysis techniques of charge transients. The nature of the defect potential is best understood by studying capture mechanisms. Carrier capture kinetics of the dominant defects are studied using variable pulse-width filling technique to directly determine capture coefficients as a function of temperature. We demonstrate that charge redistribution occurs among the discrete traps through the process of multiple trapping and emission. Of the eight deep traps detected, the two dominant ones, christened T2 and T3, with activation energies of 0.38 and 0.49 eV are studied in detail. We go on to develop a phenomenological model representing the various activation energies in the mechanism of capture and emission. One of them involves a capture barrier of as high as 0.27 eV, indicating the occurrence of multi-phonon emission mechanism during capture. There is evidence to suggest that the dominant traps are different states of a defect from the same physical origin. [ABSTRACT FROM AUTHOR]

The third (B3) and fourth (B4) virial coefficients of hard disks in narrow channels have been evaluated accurately by Gaussian quadrature for a range of channel widths (2Rp), Rp < (√3 + 2)/4 = 0.9330... (in units of the hard disk diameter). These widths correspond to the important regime of fluids in single file formations with only nearest-neighbor interactions. Accurate results are obtained by using N-point Gaussian quadrature and extrapolating to the infinite N limits. Since not much is known about these B3 and B4, our new results should provide guidance for exact calculations of the virial coefficients and benchmarks for other numerical methods. [ABSTRACT FROM AUTHOR]

Rotational excitation of the hydrogen cyanide (HCN) molecule by collisions with para-H2( j = 0, 2) and ortho-H2( j = 1) is investigated at low temperatures using a quantum time independent approach. Both molecules are treated as rigid rotors. The scattering calculations are based on a highly correlated ab initio 4-dimensional (4D) potential energy surface recently published. Rotationally inelastic cross sections among the 13 first rotational levels of HCN were obtained using a pure quantum close coupling approach for total energies up to 1200 cm-1. The corresponding thermal rate coefficients were computed for temperatures ranging from 5 to 100 K. The HCN rate coefficients are strongly dependent on the rotational level of the H2 molecule. In particular, the rate coefficients for collisions with para-H2( j = 0) are significantly lower than those for collisions with ortho-H2( j = 1) and para-H2( j = 2). Propensity rules in favor of even Δj transitions were found for HCN in collisions with para-H2( j = 0) whereas propensity rules in favor of odd Δj transitions were found for HCN in collisions with H2( j ≥ 1). The new rate coefficients were compared with previously published HCN-para-H2( j = 0) rate coefficients. Significant differences were found due the inclusion of the H2 rotational structure in the scattering calculations. These new rate coefficients will be crucial to improve the estimation of the HCN abundance in the interstellar medium. [ABSTRACT FROM AUTHOR]

Rotational excitation of the hydrogen cyanide (HCN) molecule by collisions with para-H2( j = 0, 2) and ortho-H2( j = 1) is investigated at low temperatures using a quantum time independent approach. Both molecules are treated as rigid rotors. The scattering calculations are based on a highly correlated ab initio 4-dimensional (4D) potential energy surface recently published. Rotationally inelastic cross sections among the 13 first rotational levels of HCN were obtained using a pure quantum close coupling approach for total energies up to 1200 cm-1. The corresponding thermal rate coefficients were computed for temperatures ranging from 5 to 100 K. The HCN rate coefficients are strongly dependent on the rotational level of the H2 molecule. In particular, the rate coefficients for collisions with para-H2( j = 0) are significantly lower than those for collisions with ortho-H2( j = 1) and para-H2( j = 2). Propensity rules in favor of even Δj transitions were found for HCN in collisions with para-H2( j = 0) whereas propensity rules in favor of odd Δj transitions were found for HCN in collisions with H2( j ≥ 1). The new rate coefficients were compared with previously published HCN-para-H2( j = 0) rate coefficients. Significant differences were found due the inclusion of the H2 rotational structure in the scattering calculations. These new rate coefficients will be crucial to improve the estimation of the HCN abundance in the interstellar medium. [ABSTRACT FROM AUTHOR]

The Schrödinger eigenvalue problem is solved with the imaginary time propagation technique. The separability of the Hamiltonian makes the problem suitable for the application of splitting methods. High order fractional time steps of order greater than two necessarily have negative steps and cannot be used for this class of diffusive problems. However, there exist methods which use fractional complex time steps with positive real parts which can be used with only a moderate increase in the computational cost. We analyze the performance of this class of schemes and propose new methods which outperform the existing ones in most cases. On the other hand, if the gradient of the potential is available, methods up to fourth order with real and positive coefficients exist. We also explore this case and propose new methods as well as sixth-order methods with complex coefficients. In particular, highly optimized sixth-order schemes for near integrable systems using positive real part complex coefficients with and without modified potentials are presented. A time-stepping variable order algorithm is proposed and numerical results show the enhanced efficiency of the new methods. [ABSTRACT FROM AUTHOR]

The first four perturbation coefficients in the expansion of the Helmholtz free energy in power series of the inverse of the reduced temperature for a number of potential models with hard-sphere cores plus core-softened and discontinuous tails are obtained from Monte Carlo simulations. The potential models considered include square-well, double square-well, and square-shoulder plus square-well, with different potential parameters. These simulation data are used to evaluate the performance of a traditional macroscopic compressibility approximation (MCA) for the second order coefficient and a recent coupling parameter series expansion (CPSE) for the first four coefficients. Comprehensive comparison indicates the incapability of the MCA for the second order coefficient in most non-stringent situations, and significance of the CPSE in accurately calculating these four coefficients. [ABSTRACT FROM AUTHOR]

While the Soret coefficients of binary mixtures have been widely measured in the past, here we report the first measurement of the Soret coefficient of a ternary mixture in a low gravity environment on board the International Space Station. The sample was contained in a 10 mm × 10 mm × 5 mm (w, l, h) cell and was monitored by means of a Mach-Zehnder interferometer at two wavelengths. The analyzed sample was a mixture of tetrahydronaphthalene, isobutylbenzene, and dodecane at the weight fraction of 0.1/0.8/0.1. While the lateral walls of the cell did not possess complete thermal isolation, the separation of the components in the central region of the cavity was comparable to purely diffusive behavior. The same experimental parameters have been monitored in Run7 and Run12 of the Selectable Optical Diagnostics Instrument-Diffusion and Soret Coefficient experiment in order to verify the accuracy of the setup. The similarity of the results demonstrates the repeatability of thermodiffusion experiments in a microgravity environment. There was nearly equal separation of the tetrahydronaphthalene and isobutylbenzene components in opposite directions, while dodecane experienced a weak separation in the same direction as isobutylbenzene. Finally, Fourier image processing and calculations of the transient separation of the components were used to analyze the heat transfer in the system and to measure the Soret coefficients for this ternary mixture. The successful measurements shown in this work can serve as the standard for ground experiments and for numerical modeling of hydrocarbon mixtures. [ABSTRACT FROM AUTHOR]