Atomistic Simulation

Thanks are due to Peter Qumbsch, who assisted in the preliminary planning of the symposium; to Christine Townsley, who provided invaluable administrative support at UCSB; to Margherita Chang and Lisa Fischer, who assembled the table of contents and indices; to the MRS staff, who offered excellent logistical support; and to the 1998 MRS Fall Meeting Chairs,

The adhesion of TiO 2 (anatase structure) nanoparticles to kaolinite substrate was investigated using molecular modeling. Universal force field computation, density function theory computation, and a combination of both two approaches were used. This study enabled the adhesion energy for the TiO 2 /kaolinite nanocomposite to be estimated, and revealed the preferred orientation of the TiO 2 nanoparticles on the kaolinite substrate. The results of all three levels of computation were compared in order to show that the accuracy of universal force field computations is sufficient in this context. The role of nanoparticle size and the importance of the nanoparticle-substrate bonding contribution are presented here and discussed. A comparison of the molecular modeling results with scanning electron microscopy observations showed that the results of the modeling were consistent with the experimental data, and that this approach can be used to help characterize nanocomposites of the nanoparticle/phyllosilicate substrate type.

We report here that Fermi pinning at the polysilicon/metal oxide interface causes high threshold voltages in MOSFET devices. Results indicate that pinning occurs due to the interfacial Si-Hf and Si-O-Al bonds for HfO 2 and Al 2 O 3 , respectively. Oxygen vacancies at polysilicon/HfO 2 interfaces also lead to Fermi pinning. We show that this fundamental characteristic affects the observed polysilicon depletion. In Part I, the theoretical background is reviewed and the impact of the different gate stack regions are separated out by investigating the relative threshold voltage shifts of devices with Hf-based dielectrics. The effects of the interfacial bonding are examined in Part II.

Many efforts in experimental and theory are dedicated to study the puzzling problem of glass transition. With the involvement of molecular modeling, a fresh look at this intricate phenomenon was taken. Nevertheless, the difficulty to accurately probe all the domains of times necessary to describe this process still remains. Moreover, using a full-atomic description to account for equilibrated systems has been called into question. However, such depiction offers a special regard since it could deal with small modifications in the polymer architecture. From an appropriate selection of phase space, it was shown that atomistic simulation was able to link simulated and experimental glass transition temperatures of vinylic polymers using the established Williams-Landel-Ferry equation. Consequently, atomic insight into the glass transition phenomenon can be gained from a comparison of simulated data with actual models and experimental data. In this paper, the different parameters intervening in the Vogel-Fulcher-Tamman equation stemming from the local dynamics of a series of vinylic polymers are thus compared. Their behavior yields an atomistic viewpoint of the Adam-Gibbs model. Figure 4. Fragility index, D, reported with respect to simulated T0/Tg at 1.2 × 10 12 K/min cooling rate, for backbone dynamics (b) and backbone/side-chain coupling (O); linear fits are also displayed.

We present a de novo hierarchical simulation framework for first-principles based predictive simulations of materials and their validation on high-end parallel supercomputers and geographically distributed clusters. In this framework, highend chemically reactive and non-reactive molecular dynamics (MD) simulations explore a wide solution space to discover microscopic mechanisms that govern macroscopic material properties, into which highly accurate quantum mechanical (QM) simulations are embedded to validate the discovered mechanisms and quantify the uncertainty of the solution. The framework includes an embedded divide-andconquer (EDC) algorithmic framework for the design of linear-scaling simulation algorithms with minimal bandwidth complexity and tight error control. The EDC framework also enables adaptive hierarchical simulation with automated model transitioning assisted by graph-based event tracking. A tunable hierarchical cellular decomposition parallelization framework then maps the O(N) EDC algorithms onto petaflops computers, while achieving performance tunability through a hierarchy of parameterized cell data/ computation structures, as well as its implementation using hybrid grid remote procedure call + message passing + threads programming. High-end computing platforms such as IBM BlueGene/L, SGI Altix 3000 and the NSF TeraGrid provide an excellent test grounds for the framework. On these platforms, we have achieved unprecedented scales of quantum-mechanically accurate and well validated, chemi-cally reactive atomistic simulations-1.06 billion-atom fast reactive force-field MD and 11.8 million-atom (1.04 trillion grid points) quantum-mechanical MD in the framework of the EDC density functional theory on adaptive multigridsin addition to 134 billion-atom non-reactive space-time multiresolution MD, with the parallel efficiency as high as 0.998 on 65,536 dual-processor BlueGene/L nodes. We have also achieved an automated execution of hierarchical QM/MD simulation on a grid consisting of 6 supercomputer centers in the US and Japan (in total of 150,000 processor hours), in which the number of processors change dynamically on demand and resources are allocated and migrated dynamically in response to faults. Furthermore, performance portability has been demonstrated on a wide range of platforms such as BlueGene/L, Altix 3000, and AMD Opteron-based Linux clusters. Downloaded from − − −4 of the bulk density. (c) Energy conservation in EDC-DFT based MD simulation of liquid Rb at 1400 K. The domain and buffer sizes are 16.424 and 8.212 a.u., respectively.

We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomog-raphy (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions.

Plasticity in bulk metallic glasses (BMGs), is normally dominated initially by shear transformations zones (STZ), which expand to form shear bands (SB) through the material. In order to control and thus improve the dynamics of plasticity, composition of metallic glasses has been modified in different ways. Particularly, the inclusion of crystalline nanoparticles provides obstacles to SB propagation and growth, with SB often nucleating at the interface between the BMG and the nanoparticle. This results in a reduced and more homogeneous deformation in the plastic regime. Nevertheless, to ensure lasting effects, inclusions should be stable in time, i.e. not diffuse into the surrounding amorphous material loosing the sharp transition from crystal to amorphous. In previous work we determined constitutive parameters of the Cu 46 Zr 54 metallic glass as a function of temperature, using atomistic Molecular Dynamics (MD) simulations. We will now present results for spherical face-centered cubic (FCC) Cu inclusions. Although we do not focus on the size effects of inclusions like other studies, we analyze the stability of the nanoparticles at different temperatures. During mechanical deformation under uniaxial strain of a BMG sample with inclusions, we analyze Voronoi polyhedra, and shear stress and shear strain localization to study the role of the inclusion in the mechanical properties of this composite material.

The NdCoO 3 perovskite has been investigated using a combination of atomistic simulation and experimental techniques to examine its possible use as an oxidation catalyst and/or sensor material. The sensing properties of NdCoO 3 and Nd 0.8 Sr 0.2 CoO 3 towards CO have been investigated by employing thin films deposited by means of radio-frequency (RF) magnetron sputtering onto polycrystalline Al 2 O 3 . The response of the films was monitored by performing four-probe DC-conductivity measurements. The conductivity variation induced by switching between a CO-free atmosphere (air) and a CO-rich one with the same composition of residual gas was recorded and analysed as a function of temperature; results are compared for the two samples. Simulation studies focussed on the dopant, transport and redox properties of the pure material; the results indicate that Sr and Ca on the Nd site are the most soluble dopants and that when divalent dopants are incorporated in the structure, charge compensation occurs via oxygen ion vacancies. The low activation energy for oxygen vacancy migration suggests high oxide-ion mobility through the lattice. Particular attention is paid to the electronic processes because of their importance with respect to practical applications of the material.

Molecular mechanics (MM) and molecular dynamics (MD) simulations on ten perfluoroalkyl methacrylates and four copolymers derived from methyl methacrylate (MMA) and 1,1-dihydroperfluorohendecyl methacrylate (F10MA) in different ratios have been performed to predict surface properties. 1,1-Dihydroperfluorohendecyl methacrylate, which contained highest number of fluorine atoms, exhibited lowest surface energy, a trend that is in accordance with experimental observations. Density calculations on selected perfluoroalkyl methacrylates have been performed using NPT dynamics, for which no experimental data are available. Computations were performed to obtain bulk properties like cohesive energy density and solubility parameter through MM and MD simulations in the NVT ensemble under periodic boundary conditions. From the equilibrated structures, surface energies were computed, which compared well with the experimental data reported in the literature. Surface energies of copolymers decreased with increasing number of perfluoroalkyl groups. Various components of energetic interactions have been examined in detail in order to gain a better insight into interactions between bulk structure and the film. The dominant contributions are from van der Waals and Coulombic energy terms. The computed mass density profile for thin films gave an indication whether the film is of sufficient thickness so that the interior of the film is indistinguishable from the bulk structure. The total pair correlation and bond correlation functions have been analyzed to confirm the amorphous nature of the simulated structures.

The effects of interstitial carbon on the diffusion and mechanical properties of copper and silver are studied theoretically. Semiempirical methodology, atomistic simulations, and ®rst-principles density functional schemes are combined to extract some understanding of the diffusion process and lattice reconstruction in extremely dilute interstitial Cu±C and Ag±C alloys. It is demonstrated that carbon inclusion in the host matrix leads to suf®cient non-uniform dilatation of the lattice. We also show that an account of static displacements is important in the calculations of the activation energy for the diffusion of the interstitial atoms. The ªembeddedº cluster scheme is suggested to simulate the relaxation in extremely dilute alloys. High-resolution scanning electron microscopy results are presented, which demonstrate the existence of a solid solution zone at the Cu±C interface. q

Due to their excellent thermal shock and wear resistance at high temperatures, alumina-carbon based refractories are used extensively in the steel industry. A clear understanding of factors affecting the dissolution of carbon from refractories is of crucial importance, as carbon depletion from the refractory can significantly deteriorate refractory performance and metal quality. Atomistic simulations on the alumina-graphite/liquid iron system have shown that nonwetting between alumina and liquid iron is an important factor inhibiting the penetration of liquid metal in the refractory matrix and limiting carbon dissolution. This study investigates the role played by the carbonaceous material in the dissolution of carbon from the refractory composite. Two carbonaceous materials, namely, petroleum coke and natural graphite, respectively, containing 0.35 and 5.26 pct ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration range. Using a sessile drop arrangement, carbon pickup by liquid iron from alumina-carbon mixtures was measured at 1550°C and was compared with the carbon pickup from alumina-synthetic graphite mixtures. These studies were supplemented with wettability measurements and microscopic investigations on the interfacial region. For high alumina concentrations (.40 wt pct), carbon dissolution from refractory mixtures was found to be negligible for all carbonaceous materials under investigation. Significant differences however were observed at lower alumina concentrations. Carbon dissolution from alumina-petroleum coke mixtures was much lower than the corresponding dissolution from alumina synthetic graphite-mixtures and was attributed to poor wettability of petroleum coke with liquid iron, its structural disorder, and the presence of sulfur. Very high levels of carbon dissolution, however, were observed from alumina-natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt pct alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt pct alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of material characteristics, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.

The formation of defects in bcc Mo lattice as a result of 50-keV Xe bombardment is studied via atomistic simulation with an interatomic potential developed using the force-matching ab initio based approach. The defect evolution in the cascade is described. Diffusion and interaction of interstitials and vacancies are analyzed. Only small interstitial atom clusters form directly in the cascade. Larger clusters grow only via aggregation at temperatures up to 2000 K. Stable forms of clusters demonstrate one-dimensional diffusion with a very high diffusion coefficient and escape quickly to the open surface. Point vacancies have much lower diffusivity and do not aggregate. The possibility of a large prismatic vacancy loop formation near the impact surface as a result of fast recrystallization is revealed. The mobility of the vacancy dislocation loop segments is high, however, the motion of the entire loops is strongly hindered by neighbor point defects. This paper explains the existence of the large prismatic vacancy loops and the absence of the interstitial loops in the recent experiments with ion irradiation of Mo foils.

The porosity impact on the UO 2 matrix thermomechanical properties was investigated using atomistic simulation techniques. The porosity modifies the thermal expansion coefficient and this is attributed to pore surface effects. The elastic moduli at 0 K and at finite temperature decrease with porosity, this variation being well approximated using affine functions. These results agree with other mesoscale model predictions and experimental data, showing the ability of the semiempirical potential atomistic simulations to give an overall good description of the porous UO 2 . However, the surface effects are incompletely described.

New materials with distinctive properties are arising and attracting the scientific community at regular intervals. Stiffness and strength are the important factors in determining stability and lifetime of any technological devices, but defects which are inevitable at the time of production can alter the structural properties of any engineering materials. Developing graphene with specific structural properties depends upon controlling these defects, either by removing or deliberately engineering atomic structure to gain or tailoring specific properties. In this article, a comprehensive review of defective graphene sheets with respect to its mechanical and thermal properties are presented and examined.

The reactor pressure vessel (RPV) steels used in current nuclear power plants embrittle as a consequence of the continuous irradiation with neutrons. Among other radiation effects, the experimentally observed formation of copper-rich defects is accepted to be one of the main causes of embrittlement. Therefore, an accurate description of the nucleation and growth under irradiation of these and other defects is fundamental for the prediction of the mechanical degradation that these materials undergo during operation, with a view to guarantee a safer plant life management. In this work we describe in detail the object kinetic Monte Carlo (OKMC) method that we developed, showing that it is well suited to investigate the evolution of radiation damage in simple Fe alloys (Fe, Fe-Cu) under irradiation conditions (temperature, dose and dose-rate) typical of experiments with different impinging particles and also operating conditions. The still open issue concerning the method is the determination of the mechanisms and parameters that should be introduced in the model in order to correctly reproduce the experimentally observed trends. The stateof-the-art, based on the input from atomistic simulation techniques, such as ab initio calculations, molecular dynamics (MD) and atomic kinetic Monte Carlo, is critically revised in detail and a sensitivity study on the effects of the choice of the reaction radii and the description of defect mobility is conducted. A few preliminary, but promising, results of favorable comparison with experimental observations are shown and possible further refinements of the model are briefly discussed.

We use a simple density functional approach on a diffusional time scale, to address freezing to the body-centered cubic (bcc), hexagonal close-packed (hcp), and face-centered cubic (fcc) structures. We observe faceted equilibrium shapes and diffusion-controlled layerwise crystal growth consistent with twodimensional nucleation. The predicted growth anisotropies are discussed in relation with results from experiment and atomistic simulations. We also demonstrate that varying the lattice constant of a simple cubic substrate, one can tune the epitaxially growing body-centered tetragonal structure between bcc and fcc, and observe a Mullins-Sekerka-Asaro-Tiller-Grinfeld-type instability.

Heme proteins are found in all living organisms, and perform a wide variety of tasks ranging from electron transport, to the oxidation of organic compounds, to the sensing and transport of small molecules. In this work we review the application of classical and quantum-mechanical atomistic simulation tools to the investigation of several relevant issues in heme proteins chemistry: (i) conformational analysis, ligand migration, and solvation effects studied using classical molecular dynamics simulations; (ii) electronic structure and spin state energetics of the active sites explored using quantum-mechanics (QM) methods; (iii) the interaction of heme proteins with small ligands studied through hybrid quantum mechanics-molecular mechanics (QM-MM) techniques; (iv) and finally chemical reactivity and catalysis tackled by a combination of quantum and classical tools.

In the last decade the zircon (U-Th)/He (ZHe) thermochronometer has been applied to a variety of geologic problems. Although bulk diffusion coefficients for He in zircon are available from laboratory step-heating experiments, little is known about the diffusion mechanism(s) and their dependence on the crystallographic structure of zircon. Here, we investigate the diffusion of He in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of He migration in zircon. Empirical force fields and quantum-mechanical calculations reveal that the energy barriers for He diffusion are strongly dependent on structure. The most favorable pathway for He diffusion is the [0 0 1] direction through the open channels parallel to the c-axis (DE Ã ½001 ¼ 13:4 kJ mol À1 , activation energy for tracer diffusion of a He atom along [0 0 1]). In contrast, energy barriers are higher in other directions where narrower channels for He diffusion are identified, such as [1 0 0], [1 0 1], and [1 1 0] (DE * of 44.8, 101.7, and 421.3 kJ mol À1 , respectively). Molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions, as well as the temperature dependence. Below the closure temperature of He in zircon [T c $ 180°C, Reiners P. W., Spell T. L., Nicolescu S., and Zanetti K. A. (2004) Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar-40/Ar-39 dating. Geochim. Cosmochim. Acta 68, 1857-1887], diffusion is anisotropic as He moves preferentially along the [0 0 1] direction, and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude (D [001] /D [100] $ 10 5 , at T = 25°C). Above this temperature, He atoms start to hop between adjacent [0 0 1] channels, along [1 0 0] and [0 1 0] directions (perpendicular to the c-axis). The diffusion along [1 0 0] and [0 1 0] is thermally activated, such that at higher temperatures, He diffusion in zircon becomes nearly isotropic (D [001] /D [100] $ 10, at T = 580°C). These results suggest that the anisotropic nature of He diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments. Furthermore, care should be taken when making geologic interpretations (e.g., exhumation rates, timing of cooling, etc.) from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified.

Adsorption of CO 2 and N 2 , both as single components and as binary mixtures, in three zeolites with identical chemical composition but differing pore structures (silicalite, ITQ-3, and ITQ-7) was studied using atomistic simulations. These three zeolites preferentially adsorb CO 2 over N 2 during both single-component and mixture adsorption. The CO 2 /N 2 selectivities observed in the three siliceous zeolites vary strongly as the adsorbent's crystal structure changes, with the selectivity in ITQ-3 being the largest. Our studies indicate that the different electric fields present inside zeolites with different crystal structures but identical chemical composition play an important role in the observed adsorption capacities and selectivities. The accuracy of the ideal absorbed solution theory in predicting the behavior of CO 2 /N 2 mixtures in silica zeolites based on single component adsorption data was also tested; this theory performs quite accurately for these adsorbed mixtures. † Part of the special issue "John C. Tully Festschrift"

Molecular dynamics (MD) simulations are used to study diffusion-accommodated creep deformation in nanocrystalline molybdenum, a body-centered cubic metal. In our simulations, the microstructures are subjected to constant-stress loading at levels below the dislocation nucleation threshold and at high temperatures (i.e., T > 0.75T melt ), thereby ensuring that the overall deformation is indeed attributable to atomic self-diffusion. The initial microstructures were designed to consist of hexagonally shaped columnar grains bounded by high-energy asymmetric tilt grain boundaries (GBs). Remarkably the creep rates, which exhibit a double-exponential dependence on temperature and a double power-law dependence on grain size, indicate that both GB diffusion in the form of Coble creep and lattice diffusion in the form of Nabarro-Herring creep contribute to the overall deformation. For the first time in an MD simulation, we observe the formation and emission of vacancies from high-angle GBs into the grain interiors, thus enabling bulk diffusion.

A suite of scalable atomistic simulation programs has been developed for materials research based on space-time multiresolution algorithms. Design and analysis of parallel algorithms are presented for molecular dynamics (MD) simulations and quantum-mechanical (QM) calculations based on the density functional theory. Performance tests have been carried out on 1,088processor Cray T3E and 1,280-processor IBM SP3 computers. The linear-scaling algorithms have enabled 6.44-billion-atom MD and 111,000-atom QM calculations on 1,024 SP3 processors with parallel efficiency well over 90%. The production-quality programs also feature wavelet-based computational-space decomposition for adaptive load balancing, spacefilling-curve-based adaptive data compression with user-defined error bound for scalable I/O, and octree-based fast visibility culling for immersive and interactive visualization of massive simulation data.

A theoretical analysis based on atomistic simulations is presented of both the semicoherent interface structure, which is formed to relax the strain due to lattice mismatch during InAs/GaAs(111)A heteroepitaxy, and the energetics of interface formation. The semicoherent interface consists of a network of intersecting misfit dislocations, which includes both perfect 90°dislocations along 112 directions and Shockley partials along 110 directions that bound faulted interfacial regions. This interface structure becomes stable for film thicknesses greater than four monolayers. Our simulation predictions of the interface structure and the critical film thickness for full strain relief are in excellent agreement with recent experimental data.

A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations.

For hot rolling applications it is essential to have materials with excellent high-temperature properties. Silicon nitride combines the properties optimum relevant to such high demanding applications, including thermal shock resistance, low density, high elastic modulus and low coefficient of friction. However, its chemical stability and corrosion resistance when brought into contact with copper at high temperatures and pressures is still a topic of research. In this study the corrosion and wear behavior of silicon nitride applied in rolling copper wires is investigated. For this purpose laboratory-scale wire rolling experiments were carried out and a series of atomistic simulations based on density functional theory DFT calculations were performed.

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 h110i tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

Molecular dynamics (MD) simulations of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C n MPy][Tf 2 N], n = 3, 4, 6, 8, 10) were conducted using an all-atom model. Radial distribution functions (RDF) were computed and structure functions were generated to compare with new X-ray scattering experimental results, reported herein. The scattering peaks in the structure functions generally shift to lower Q values with increased temperature for all the liquids in this series. However, the first sharp diffraction peak (FSDP) in the longer alkyl chain liquids displays a marked shift to higher Q values with increasing temperature. Alkyl chain-dependent ordering of the polar groups and increased tail aggregation with increasing alkyl chain length were observed in the partial pair correlation functions and the structure functions. The reasons for the observed alkyl chaindependent phenomena and temperature effects were explored.

AbstractÐAtomistic simulations oer an important route towards understanding and modeling materials behavior. Incorporating the essential physics into the models of interatomic interactions is increasingly dif-®cult as materials with more complex electronic structures than f.c.c. transition metals are addressed. For b.c.c. metals, interatomic potentials have been developed that incorporate angularly dependent interactions to accommodate the physics of partially ®lled d-bands. A good test of these new models is to predict the structure of crystal defects and compare them with experimentally observed defect structures. To that end, the S5 310a001 symmetric tilt grain boundary in Mo has been fabricated and characterized by HREM. The experimentally observed structure is found to agree with predictions based on atomistic simulations using angular-force interatomic potentials developed from model generalized pseudopotential theory (MGPT), but disagrees with predictions based on radial-force potentials, such as those obtained from the Finnis±Sinclair method or the embedded atom method (EAM). #

An interatomic potential for zinc oxide and its elemental constituents is derived based on an analytical bond-order formalism. The model potential provides a good description of the bulk properties of various solid structures of zinc oxide including cohesive energies, lattice parameters, and elastic constants. For the pure elements zinc and oxygen the energetics and structural parameters of a variety of bulk phases and in the case of oxygen also molecular structures is reproduced. The dependence of thermal and point defect properties on the cutoff parameters is discussed. As exemplary applications the irradiation of bulk zinc oxide and the elastic response of individual nanorods are studied.

A detailed continuum (mean-field) model is presented that captures quantitatively the evolution of a vacancy cluster size distribution in crystalline silicon simulated directly by large-scale parallel molecular dynamics. The continuum model is parameterized entirely using the results of atomistic simulations based on the same empirical potential used to perform the atomistic aggregation simulation, leading to an internally consistent comparison across the two scales. It is found that an excellent representation of all measured components of the cluster size distribution can be obtained with consistent parameters only if the assumed physical mechanisms are captured correctly. In particular, the inclusion of vacancy cluster diffusion and a model to capture the dynamic nature of cluster morphology at high temperature are necessary to reproduce the results of the large-scale atomistic simulation. Dynamic clusters with large capture volumes at high temperature, which are the result of rapid cluster shape fluctuations, are shown to be larger than would be expected from static analyses, leading to substantial enhancement of the nucleation rate. Based on these results, it is shown that a parametrically consistent atomistic-continuum comparison can be used as a sensitive framework for formulating accurate continuum models of complex phenomena such as defect aggregation in solids.

By multimillion-atom classical molecular dynamics simulations employing an embedded atom method potential, we investigate shock-induced phase transformations in body-centered cubic Fe single crystals caused by shock loading along ͓001͔ bcc , ͓011͔ bcc , and ͓111͔ bcc directions. Significant dependence of the developing microstructure on the crystallographic shock direction is evident, but we see only a weak dependence of the transition pressures for the body-centered→ close-packed and solid→ melt transitions on the shock direction. The Hugoniots obtained by simulations of samples with lengths approaching one micrometer are compared to experimental work for pressures and temperatures above shock-induced melting. Crystallographic relationships between the parent and product phase found in the simulations are common for martensitic transformations. We discuss the influence of different embedded atom method potentials on the dynamics of the transformation. We see solitary waves ahead of the shock front. The velocities of these waves decrease in time, such that they are absorbed into the shock front within a distance of propagation of one m or less.

Some of the most effective corrosion inhibitors for oil field pipeline applications are the oleic imidazoline (OI) class of molecules. However, the mechanism by which OIs inhibit corrosion is not known. We report atomistic simulations (quantum mechanics and molecular dynamics) designed to elucidate this mechanism. These studies lead to the self-assembled monolayer (SAM) model for corrosion inhibition, which explains the differences in corrosion inhibition efficiency for various OI molecules. The SAM model of OI inhibitors involves the following critical elements: (i) The function of the OI is to form a self-assembled monolayer on the native oxide surface of iron; this serves a protective role by forming a hydrophobic barrier preventing migration of H 2O, O2, and electrons to the Fe surface. (ii) The imidazoline head group serves as a sufficiently strong Lewis base to displace H2O from the Lewis acid sites of the iron oxide surface. (iii) These head groups self-assemble on the surface to form an ordered monolayer on the iron oxide surface. [ 3 × 3 for the (001) cleavage surface of R-Fe2O3.] (iv) The long hydrophobic tail (e.g., 2-oleic acid) tilts to form a tightly packed hydrophobic monolayer. [For R-Fe2O3(001) the tilt angle is about 72°with respect to the surface normal.] (v) This hydrocarbon tail must have a sufficient length to cover the surface. [For R-Fe2O3-(001) the chain length must be 12 or more carbon atoms.] (vi) The hydrophobic tail and the pendent group (e.g., -CH2CH2NH2) must lead to an octanol/water partition coefficient (log P) below a critical value in order to rapidly form the monolayer. This SAM model should be useful in developing both alternative environmentally benign corrosion inhibitors and higher temperature corrosion inhibitors. Osborne, C.; Webster, S.; Klenerman, D.; Joseph, M.; Ostovar, P.; Doyle, M. Corros.

The crystal structures and bulk moduli of the known or proposed TiO 2 polymorphs (which include the low-pressure forms rutile, anatase, brookite, TiO 2 (B), TiO 2 (H) and TiO 2 (R), and the high-pressure columbite-, baddeleyite-, cotunnite-, pyrite-, and uorite-structured forms) have been simulated using static lattice energy minimisation and two independent force®elds, namely a variable charge Morse potential and a ®xed charge rigid ion potential. The results obtained demonstrate that the variable charge model is capable of describing the crystal structures of all the polymorphs with Ti±O coordination ranging from six to eight, but fails for the nine-coordinated cotunnite structure. The ®xed-charge model is successful in predicting the crystal structures of most of the polymorphs including the cotunnite-type, but fails for the ramsdellite-structured TiO 2 (R) and is poor for baddeleyite-type TiO 2. The variable charge model is more successful in predicting the bulk moduli of the low-pressure polymorphs, whereas the ®xed charge model is marginally better in predicting the bulk moduli of the high-pressure phases. The relative phase stabilities of the low-pressure phases and the high-pressure phase diagram obtained with the variable charge model are also in broad qualitative agreement with available calorimetric, phase equilibrium, and simulation data.

A Green's function technique is developed for the relaxation of simulation cell boundaries in the modelling of dislocation interactions using molecular dy- namics. This method allows the replacement of fixed or periodical boundary conditions with flexible bound- ary conditions, thus minimizing the artificial effects due to images forces introduced by the fixed boundary con- dition, or the periodic repetition of

We have carried out an atomic-level molecular dynamics simulation of a system of nanoscopic size containing a domain of 18:0 sphingomyelin and cholesterol embedded in a fully hydrated dioleylposphatidylcholine (DOPC) bilayer. To analyze the interaction between the domain and the surrounding phospholipid, we calculate order parameters and area per molecule as a function of molecule type and proximity to the domain. We propose an algorithm based on Voronoi tessellation for the calculation of the area per molecule of various constituents in this ternary mixture. The calculated areas per sphingomyelin and cholesterol are in agreement with previous simulations. The simulation reveals that the presence of the liquid-ordered domain changes the packing properties of DOPC bilayer at a distance as large as ∼8 nm. We calculate electron density profiles and also calculate the difference in the thickness between the domain and the surrounding DOPC bilayer. The calculated difference in thickness is consistent with data obtained in atomic force microscopy experiments.

Nanoscience is an area with increasing interest both in the physicochemical phenomena involved and the potential applications such as silicon carbide films, carbon nanotubes, quantum dots, MEMS etc. These materials exhibit very interesting properties (electronic, optical, mechanical) at various length/time scales necessitating better insight. Modern fabrication techniques, such as CVD, also require better understanding in a wide range of length/time scales, in order to achieve better process control. Multiscale modeling is a new, fast developing and challenging scientific field with contributions from many scientific disciplines in an effort to assure materials simulation across length/time scales. In this paper we present a brief review of recent advances in multiscale materials modeling. First, a classification of existing simulation methods based on time and length scales is presented along with basic principles of the multiscale approach. More specifically, we focus on electronic structure calculations, classical atomistic simulation with molecular dynamics or monte carlo methods at the nano/micro scale, Kinetic Monte Carlo for larger system/ time scales and finite elements for very large scales. Then, we present the hierarchical and the hybrid strategies of multiscale modeling to couple these methods. Finally, we deal with selected applications concerning thin film CVD deposition and mechanical behavior of carbon nanotubes and we conclude presenting an overview of future trends of multiscale modeling.

This work is concerned with the rationalization and prediction of solvent and temperature effects in nucleophilic addition to alpha-chiral carbonyl compounds leading to facial diastereoselectivity. We study, using molecular dynamics simulations, the facial solvation of (R)-2-phenyl-propionaldehyde in n-pentane and n-octane at a number of temperatures and compare it with experimental selectivity data for the nBuLi addition leading to syn- and anti-(2R)-2-phenyl-3-heptanol, which give nonlinear Eyring plots with the presence of inversion temperatures. We have found from simulations that the facial solvation changes with temperature and alkane. Moreover, by introducing a suitable molecular chirality index we have been able to predict break temperatures (T(CI)) for the two solvents within less than 20 degrees of the inversion temperatures experimentally observed in the diastereoselective nBuLi addition. We believe this could lead to a viable approach for predicting inversion temperature...

A new simulation code for modelling extended defects e.g. linear (dislocations) and planar (surfaces and grain boundaries) at the atomistic level is introduced. One of the key components is the ability to calculate the Coulombic potential of a solid with one-dimensional periodicity. This approach has been applied to screw dislocations in MgO and we have evaluated the structure (including core size) and stability of the (100) and 1/2( 110) screw dislocations. The 1/2( 110) dislocation, which has the shortest Burgers vector, was found to be more stable, as predicted by elasticity theory, although the simulations show that elasticity theory underestimates the energy difference.