The distance fluctuation criterion for melting: Comparison of square-well and Morse potential models for clusters and homopolymers

Effect of composition and architecture on the thermodynamic behavior of AuCu nanoparticles

The chemical and physical properties of nanomaterials ultimately rely on their crystal structures, chemical compositions and distributions. In this paper, a series of AuCu bimetallic nanoparticles with well-defined architectures and variable compositions has been addressed to explore their thermal stability and thermally driven behavior by molecular dynamics simulations. By combination of energy and Lindemann criteria, the solid-liquid transition and its critical temperature were accurately identified. Meanwhile, atomic diffusion, bond order, and particle morphology were examined to shed light on thermodynamic evolution of the particles. Our results reveal that composition-dependent melting point of AuCu nanoparticles significantly departs from the Vegard's law prediction. Especially, chemically disordered (ordered) alloy nanoparticles exhibited markedly low (high) melting points in comparison with their unary counterparts, which should be attributed to enhancing (decreasing) atomic diffusivity in alloys. Furthermore, core-shell structures and heterostructures demonstrated a mode transition between the ordinary melting and the two-stage melting with varying Au content. AuCu alloyed nanoparticles presented the evolution tendency of chemical ordering from disorder to order before melting and then to disorder during melting. Additionally, as the temperature increases, the shape transformation was observed in AuCu nanoparticles with heterostructure or L10 structure owing to the difference in thermal expansion coefficients of elements and/or of crystalline orientations. Our findings advance the fundamental understanding on thermodynamic behavior and stability of metallic nanoparticles, offering theoretical insights for design and application of nanosized particles with tunable properties.

Atomistically informed hierarchical modeling for revisiting the constituent structures from heredity and nano-micro mechanics of sheath-core carbon fiber

To better understand the heterogeneous anisotropic nanocomposite features and provide reliable underlying constitutive parameters of carbon fiber for continuum-level simulations, hierarchical modeling approaches combining quantum chemistry, molecular dynamics, numerical and analytical micromechanics are employed for studying the structure-performance relationships of the precursor-inherited sheath-core carbon fiber layers. A robust debonding force field is derived from energy matching protocols, including bond dissociation enthalpy calculations and rigid-constraint potential energy surface scan. Logistic long range bond stretching curves with exponential parameters and shifted force vdW curves are designed to diminish energy perturbations. The pseudo-crystalline microstructure is proposed and validated using virtual wide angle X-ray diffraction patterns and bond-orientational order parameters. The distribution or alignment features of the nanocomposite microstructures are collected from quantum chemical topology analysis and normal vector extractions. Non-equilibrium tensile loading simulation predicts the decomposed strain energy contributions, principal-axis modulus, strength limit, localized stress, and fracture morphologies of the model. Finally, an atomistically-informed stiffness prediction model combining numerical homogenization and analytical self-consistent Eshelby-Mori-Tanaka-type effective mean field micromechanics theory is proposed, giving a successful estimation of the overall stiffness matrix of the sheath-core carbon fiber system. The hierarchical models in combination with the carbonization reaction template will help in providing efficient and feasible schemes for the synergistic process-performance control of distinct types of carbon fiber.

Size-dependent melting of onion-like fullerenic carbons: a molecular dynamics and machine learning study

The melting thermodynamic characteristics of 2- to 20-layered onion-like fullerenes (OLF_n) (C₆₀@C₂₄₀to C₆₀@···@C₆₀₀₀···@C₂₄₀₀₀) are comprehensively explored using first-principles-based ReaxFF atomistic simulations and random forest machine learning (RF ML). It is revealed that OLF_nshows lower thermal stability than the counterparts of single-walled fullerenes (SWF_n). The melting point of SWF_nincreases monotonically with increasing size, whereas for OLF_n, an unusual size-dependent melting point is observed; OLF_nwith intermediate size shows the highest melting point. For small OLF_n, the melting occurs from the inner to the outer, whereas for large OLF_n, it nucleates from the inner to the outer and to intermediate fullerenes. The melting and erosion behaviors of both SWF_nand OLF_nare mainly characterized by the nucleation of non-hexagons, nanovoids, carbon chains and emission of C₂. RF ML model is developed to predict the melting points of both SWF_nand OLF_n. Moreover, the analysis of the feature importance reveals that the Stone-Wales transformation is a critical pathway in the melting of SWF_nand OLF_n. This study provides new insights and perspectives into the thermodynamics and pyrolysis chemistry of fullerenic carbons, and also may shed some lights onto the understanding of thermally-induced erosion of carbon-based resources and spacecraft materials.

Self-sustained non-equilibrium co-existence of fluid and solid states in a strongly coupled complex plasma system

A complex (dusty) plasma system is well known as a paradigmatic model for studying the kinetics of solid-liquid phase transitions in inactive condensed matter. At the same time, under certain conditions a complex plasma system can also display characteristics of an active medium with the micron-sized particles converting energy of the ambient environment into motility and thereby becoming active. We present a detailed analysis of the experimental complex plasmas system that shows evidence of a non-equilibrium stationary coexistence between a cold crystalline and a hot fluid state in the structure due to the conversion of plasma energy into the motion energy of microparticles in the central region of the system. The plasma mediated non-reciprocal interaction between the dust particles is the underlying mechanism for the enormous heating of the central subsystem, and it acts as a micro-scale energy source that keeps the central subsystem in the molten state. Accurate multiscale simulations of the system based on combined molecular dynamics and particle-in-cell approaches show that strong structural nonuniformity of the system under the action of electostatic trap makes development of instabilities a local process. We present both experimental tests conducted with a complex plasmas system in a DC glow discharge plasma and a detailed theoretical analysis.

Thermally activated microstructural evolution of metallic heterophase nanoparticles: insights from molecular dynamics simulations

A crystal phase is a key factor to determine the physical and chemical properties of crystalline materials. As a new class of nanoscale structures, heterophase nanoparticles, which assemble conventional and unconventional phases, exhibit exceptional properties in comparison with their single-phase counterparts. In this work, we explored the thermodynamic stability of Au, Co, and AuCo heterophase nanoparticles with fcc and hcp phases by using molecular dynamics simulations. These heterostructured nanoparticles were continuously heated to examine their thermally activated structural evolutions. Au and Co single-phase nanoparticles were also considered for comparison. The results show that the phase transition between fcc and hcp is absent in these heterophase nanoparticles despite the existence of an unconventional phase. Although the melting of Au and Co heterophase nanoparticles is homogeneous, AuCo heterophase nanoparticles show heterogeneous melting, i.e., the Au fcc domain firstly melts, followed by the melting of the Co hcp domain, exhibiting a typical two-stage melting characteristic and resulting in the existence of a solid-core/liquid-shell structure within a considerable temperature region. Furthermore, the mutual diffusion of atoms between fcc and hcp domains is observed in the Au and Co heterophase nanoparticles. However, the unidirectional diffusion from the Au domain to the Co domain is found in the AuCo heterophase nanoparticles prior to their overall melting. This study deepens the fundamental understanding of the thermodynamic evolution of metallic heterogeneous nanoparticles and provides mechanistic and quantitative guidance for the rational design and applications of nanoscale multiphase heterostructures.

Noble metal alloy thin films by atomic layer deposition and rapid Joule heating

Metal alloys are usually fabricated by melting constituent metals together or sintering metal alloy particles made by high energy ball milling (mechanical alloying). All these methods only allow for bulk alloys to be formed. This manuscript details a new method of fabricating Rhodium-Iridium (Rh-Ir) metal alloy films using atomic layer deposition (ALD) and rapid Joule heating induced alloying that gives functional thin film alloys, enabling conformal thin films with high aspect ratios on 3D nanostructured substrate. In this work, ALD was used to deposit Rh thin film on an Al2O3 substrate, followed by an Ir overlayer on top of the Rh film. The multilayered structure was then alloyed/sintered using rapid Joule heating. We can precisely control the thickness of the resultant alloy films down to the atomic scale. The Rh-Ir alloy thin films were characterized using scanning and transmission electron microscopy (SEM/TEM) and energy dispersive spectroscopy (EDS) to study their microstructural characteristics which showed the morphology difference before and after rapid Joule heating and confirmed the interdiffusion between Rh and Ir during rapid Joule heating. The diffraction peak shift was observed by Grazing-incidence X-ray diffraction (GIXRD) indicating the formation of Rh-Ir thin film alloys after rapid Joule heating. X-ray photoelectron spectroscopy (XPS) was also carried out and implied the formation of Rh-Ir alloy. Molecular dynamics simulation experiments of Rh-Ir alloys using Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) were performed to elucidate the alloying mechanism during the rapid heating process, corroborating the experimental results.

Mean-field model of melting in superheated crystals based on a single experimentally measurable order parameter

Melting is one of the most studied phase transitions important for atomic, molecular, colloidal, and protein systems. However, there is currently no microscopic experimentally accessible criteria that can be used to reliably track a system evolution across the transition, while providing insights into melting nucleation and melting front evolution. To address this, we developed a theoretical mean-field framework with the normalised mean-square displacement between particles in neighbouring Voronoi cells serving as the local order parameter, measurable experimentally. We tested the framework in a number of colloidal and in silico particle-resolved experiments against systems with significantly different (Brownian and Newtonian) dynamic regimes and found that it provides excellent description of system evolution across melting point. This new approach suggests a broad scope for application in diverse areas of science from materials through to biology and beyond. Consequently, the results of this work provide a new guidance for nucleation theory of melting and are of broad interest in condensed matter, chemical physics, physical chemistry, materials science, and soft matter.

Multivalent Ion-Mediated Attraction between Like-Charged Colloidal Particles: Nonmonotonic Dependence on the Particle Charge

Ion-mediated effective interactions are important for the structure and stability of charged particles such as colloids and nucleic acids. It has been known that the intrinsic electrostatic repulsion between like-charged particles can be modulated into effective attraction by multivalent ions. In this work, we examined the dependence of multivalent ion-mediated attraction between like-charged colloidal particles on the particle charge in a wide range by extensive Monte Carlo simulations. Our calculations show that for both divalent and trivalent salts, the effective attraction between like-charged colloidal particles becomes stronger with the increase of the particle charge, whereas it gradually becomes weakened when the particle charge exceeds a "critical" value. Correspondingly, as the particle charge is increased, the driving force for such effective attraction transits from an attractive electrostatic force to an attractive depletion force, and the attraction weakening by high particle charges is attributed to the transition of electrostatic force from attraction to repulsion. Our analyses suggest that the attractive depletion force and the repulsive electrostatic force at high particle charges result from the Coulomb depletion which suppresses the counterion condensation in the limited region between two like-charged colloidal particles. Moreover, our extensive calculations indicate that the "critical" particle charge decreases apparently for larger ions and smaller colloidal particles due to stronger Coulomb depletion and decreases slightly at higher salt concentrations due to the slightly enhanced Coulomb depletion in the intervening space between colloidal particles. Encouragingly, we derived an analytical formula for the "critical" particle charge based on the Lindemann melting law.

Molecular Dynamics Simulations of Melting Iron Nanoparticles with/without Defects Using a Reaxff Reactive Force Field

Molecular dynamics simulations are performed to study thermal properties of bulk iron material and Fe nanoparticles (FNP) by using a ReaxFF reactive force field. Thermodynamic and energy properties such as radial distribution function, Lindemann index and potential energy plots are adopted to study the melting behaviors of FNPs from 300 K to 2500 K. A step-heating method is introduced to obtain equilibrium melting points. Our results show ReaxFF force field is able to detect size effect in FNP melting no matter in energy or structure evolution aspect. Extra storage energy of FNPs caused by defects (0%-10%) is firstly studied in this paper: defects will not affect the melting point of FNPs directly but increase the system energy especially when temperature reaches the melting points.

The folding pathways and thermodynamics of semiflexible polymers

Inspired by the protein folding and DNA packing, we have systematically studied the thermodynamic and kinetic behaviors of single semiflexible homopolymers by Langevin dynamics simulations. In line with experiments, a rich variety of folding products, such as rod-like bundles, hairpins, toroids, and a mixture of them, are observed in the complete diagram of states. Moreover, knotted structures with a significant population are found in a certain range of bending stiffness in thermal equilibrium. As the solvent quality becomes poorer, the population of the intermediate occurring in the folding process increases, which leads to a severe chevron rollover for the folding arm. However, the population of the intermediates in the unfolding process is very low, insufficient to induce unfolding arm rollover. The total types of folding pathways from the coil state to the toroidal state for a semiflexible polymer chain remain unchanged by varying the solvent quality or temperature, whereas the kinetic partitioning into different folding events can be tuned significantly. In the process of knotting, three types of mechanisms, namely, plugging, slipknotting, and sliding, are discovered. Along the folding evolution, a semiflexible homopolymer chain can knot at any stage of folding upon leaving the extended coil state, and the probability to find a knot increases with chain compactness. In addition, we find rich types of knotted topologies during the folding of a semiflexible homopolymer chain. This study should be helpful in gaining insight into the general principles of biopolymer folding.

Physisorption of H2 on Fullerenes and the Solvation of C60 by Hydrogen Clusters at Finite Temperature: A Theoretical Assessment

The interaction between hydrogen and carbonaceous nanostructures is of fundamental interest in various areas of physical chemistry. In this contribution we have revisited the physisorption of hydrogen molecules and H₂ clusters on fullerenes, following a first-principles approach in which the interaction is quantitatively evaluated for the C₂₀ system using high-level electronic structure methods. Relative to coupled cluster data at the level of single, double, and perturbative triple excitations taken as a benchmark, the results for rotationally averaged physisorbed H₂ show a good performance of MP2 variants and symmetry-adapted perturbation theory, but significant deviations and basis set convergence issues are found for dispersion-corrected density functional theory. These electronic structure data are fitted to produce effective coarse-grained potentials for use in larger systems such as C₆₀-H₂. Using path-integral molecular dynamics, the potentials are also applied to parahydrogen clusters solvated around fullerenes, across the regime where the first solvation shell becomes complete and as a function of increasing temperature. For C₆₀ our findings indicate a sensible dependence of the critical solvation size on the underlying potential. As the temperature is increased, a competition is found between the surface and radial expansions of the solvation shell, with one molecule popping away at intermediate temperatures but getting reinserted at even higher temperatures.

Structure, thermodynamics, and rearrangement mechanisms in gold clusters-insights from the energy landscapes framework

We consider finite-size and temperature effects on the structure of model AuN clusters (30 ≤ N ≤ 147) bound by the Gupta potential. Equilibrium behaviour is examined in the harmonic superposition approximation, and the size-dependent melting temperature is also bracketed using molecular dynamics simulations. We identify structural transitions between distinctly different morphologies, characterised by various defect features. Reentrant behaviour and trends with respect to cluster size and temperature are discussed in detail. For N = 55, 85, and 147 we visualise the topography of the underlying potential energy landscape using disconnectivity graphs, colour-coded by the cluster morphology; and we use discrete path sampling to characterise the rearrangement mechanisms between competing structures separated by high energy barriers (up to 1 eV). The fastest transition pathways generally involve metastable states with multiple fivefold disclinations and/or a high degree of amorphisation, indicative of melting. For N = 55 we find that reoptimising low-lying minima using density functional theory (DFT) alters their energetic ordering and produces a new putative global minimum at the DFT level; however, the equilibrium structure predicted by the Gupta potential at room temperature is consistent with previous experiments.

Size-dependent melting modes and behaviors of Ag nanoparticles: a molecular dynamics study

The size-dependent melting behaviors and mechanisms of Ag nanoparticles (NPs) with diameters of 3.5-16 nm were investigated by molecular dynamics (MD). Two distinct melting modes, non-premelting and premelting with transition ranges of about 7-8 nm, for Ag NPs were demonstrated via the evolution of distribution and transition of atomic physical states during annealing. The small Ag NPs (3.5-7 nm) melt abruptly without a stable liquid shell before the melting point, which is characterized as non-premelting. A solid-solid crystal transformation is conducted through the migration of adatoms on the surface of Ag NPs with diameters of 3.5-6 nm before the initial melting, which is mainly responsible for slightly increasing the melting point of Ag NPs. On the other hand, surface premelting of Ag NPs with diameters of 8-16 nm propagates from the outer shell to the inner core with initial anisotropy and late isotropy as the temperature increases, and the close-packed facets {111} melt by a side-consumed way which is responsible for facets {111} melting in advance relative to the crystallographic plane {111}. Once a stable liquid shell is formed, its size-independent minimum thickness is obtained, and a three-layer structure of atomic physical states is set up. Lastly, the theory of point defect-pair (vacancy-interstitial) severing as the mechanism of formation and movement of the solid-liquid interface was also confirmed. Our study provides a basic understanding and theoretical guidance for the research, production and application of Ag NPs.

Structure and Thermodynamics of Metal Clusters on Atomically Smooth Substrates

We analyze the structure of model Ni_N and Cu_N clusters (N = 55, 147) supported on a variety of atomically smooth van der Waals surfaces. The global minima are mapped in the space of two parameters: (i) the laterally averaged surface stickiness, γ, which controls the macroscopic wetting angle, and (ii) the surface microstructure, which produces more subtle but important templating via epitaxial stresses. We find that adjusting the substrate lattice (even at constant γ) can favor different crystal plane orientations in the cluster, stabilize hexagonal close-packed order, or induce various defects, such as stacking faults, twin boundaries, and five-fold disclinations. Thermodynamic analysis reveals substrate-dependent solid-solid transitions in cluster morphology, with tunable transition temperature and sometimes exhibiting re-entrant behavior. These results shed new light on the extent to which a supporting surface can be used to influence the equilibrium behavior of nanoparticles.

Thermal Stability of Co-Pt and Co-Au Core-Shell Structured Nanoparticles: Insights from Molecular Dynamics Simulations

Co-Pt and Co-Au core-shell nanoparticles were heated by molecular dynamics simulations to investigate their thermal stability. Two core structures, that is, hcp Co and fcc Co, have been addressed. The results demonstrate that the hcp-fcc phase transition happens in the hcp-Co-core/fcc-Pt-shell nanoparticle, while it is absent in the hcp-Co-core/fcc-Au-shell one. The stacking faults appear in both Pt and Au shells despite different structures of the Co core. The Co core and Pt shell concurrently melt and present an identical melting point in both Co-Pt core-shell nanoparticles. However, typical two-stage melting occurs in both Co-Au core-shell nanoparticles. Furthermore, the Au shell in the hcp-Co-core/fcc-Au-shell nanoparticle exhibits a lower melting point than that in the fcc-Co-core/fcc-Au-shell one, while the melting points are closely equal for both hcp and fcc Co cores. All of these observations suggest that their thermal stability strongly depends on the structure of the core and the element of the shell.

First principles study of bimetallic Ni13-nAgn nano-clusters (n = 0-13): Structural, mixing, electronic, and magnetic properties

Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni_13-nAg_n nano-clusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg₁₂ alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with the Ag environment causes reduction of Ni moment.

Thermal Stability of Platinum-Cobalt Bimetallic Nanoparticles: Chemically Disordered Alloys, Ordered Intermetallics, and Core-Shell Structures

Pt-Co bimetallic nanoparticles are promising candidates for Pt-based nanocatalysts and magnetic-storage materials. By using molecular dynamics simulations, we here present a detailed examination on the thermal stabilities of Pt-Co bimetallic nanoparticles with three configurations including chemically disordered alloy, ordered intermetallics, and core-shell structures. It has been revealed that ordered intermetallic nanoparticles possess better structural and thermal stability than disordered alloyed ones for both Pt₃Co and PtCo systems, and Pt₃Co-Pt core-shell nanoparticles exhibit the highest melting points and the best thermal stability among Pt-Co bimetallic nanoparticles, although their meltings all initiate at the surface and evolve inward with increasing temperatures. In contrast, Co-Pt core-shell nanoparticles display the worst thermal stability compared with the aforementioned nanoparticles. Furthermore, their melting initiates in the core and extends outward surface, showing a typical two-stage melting mode. The solid-solid phase transition is discovered in Co core before its melting. This work demonstrates the importance of composition distribution to tuning the properties of binary nanoparticles.

QM/MD simulations on the role of SiO2 in polymeric insulation materials

SiO₂ is an efficient electron-injection material and can help stabilize polyethylene in electric insulation materials.

Large-area 2D microgel colloidal crystals fabricated via benzophenone-based photochemical reaction

Large area, high quality 2D microgel CCs were fabricated by first assembling the microgel spheres into highly ordered 3D CCs, followed by fixing the first 111 plane of the 3D CC onto the substrate via a benzophenone-based photochemical reaction.

Crystalline Ni3C as both carbon source and catalyst for graphene nucleation: a QM/MD study

Graphene nucleation from crystalline Ni₃C has been investigated using quantum chemical molecular dynamics (QM/MD) simulations based on the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. It was observed that the lattice of Ni₃C was quickly relaxed upon thermal annealing at high temperature, resulting in an amorphous Ni₃C catalyst structure. With the aid of the mobile nickel atoms, inner layer carbon atoms precipitated rapidly out of the surface and then formed polyyne chains and Y-junctions. The frequent sinusoidal-like vibration of the branched carbon configurations led to the formation of nascent graphene precursors. In light of the rapid decomposition of the crystalline Ni₃C, it is proposed that the crystalline Ni₃C is unlikely to be a reaction intermediate in the CVD-growth of graphene at high temperatures. However, results present here indicate that Ni₃C films can be employed as precursors in the synthesis of graphene with exciting possibility.

Stationary solutions for the nonlinear Schrödinger equation modeling three-dimensional spherical Bose-Einstein condensates in general potentials

Stationary solutions for the cubic nonlinear Schrödinger equation modeling Bose-Einstein condensates (BECs) confined in three spatial dimensions by general forms of a potential are studied through a perturbation method and also numerically. Note that we study both repulsive and attractive BECs under similar frameworks in order to deduce the effects of the potentials in each case. After outlining the general framework, solutions for a collection of specific confining potentials of physical relevance to experiments on BECs are provided in order to demonstrate the approach. We make several observations regarding the influence of the particular potentials on the behavior of the BECs in these cases, comparing and contrasting the qualitative behavior of the attractive and repulsive BECs for potentials of various strengths and forms. Finally, we consider the nonperturbative where the potential or the amplitude of the solutions is large, obtaining various qualitative results. When the kinetic energy term is small (relative to the nonlinearity and the confining potential), we recover the expected Thomas-Fermi approximation for the stationary solutions. Naturally, this also occurs in the large mass limit. Through all of these results, we are able to understand the qualitative behavior of spherical three-dimensional BECs in weak, intermediate, or strong confining potentials.

Anomalous diffusion in neutral evolution of model proteins

Protein evolution is frequently explored using minimalist polymer models, however, little attention has been given to the problem of structural drift, or diffusion. Here, we study neutral evolution of small protein motifs using an off-lattice heteropolymer model in which individual monomers interact as low-resolution amino acids. In contrast to most earlier models, both the length and folded structure of the polymers are permitted to change. To describe structural change, we compute the mean-square distance (MSD) between monomers in homologous folds separated by n neutral mutations. We find that structural change is episodic, and, averaged over lineages (for example, those extending from a single sequence), exhibits a power-law dependence on n. We show that this exponent depends on the alignment method used, and we analyze the distribution of waiting times between neutral mutations. The latter are more disperse than for models required to maintain a specific fold, but exhibit a similar power-law tail.

Tunable thermodynamic stability of Au-CuPt core-shell trimetallic nanoparticles by controlling the alloy composition: insights from atomistic simulations

A microscopic understanding of the thermal stability of metallic core-shell nanoparticles is of importance for their synthesis and ultimately application in catalysis. In this article, molecular dynamics simulations have been employed to investigate the thermodynamic evolution of Au-CuPt core-shell trimetallic nanoparticles with various Cu/Pt ratios during heating processes. Our results show that the thermodynamic stability of these nanoparticles is remarkably enhanced upon rising Pt compositions in the CuPt shell. The melting of all the nanoparticles initiates at surface and gradually spreads into the core. Due to the lattice mismatch among Au, Cu and Pt, stacking faults have been observed in the shell and their numbers are associated with the Cu/Pt ratios. With the increasing temperature, they have reduced continuously for the Cu-dominated shell while more stacking faults have been produced for the Pt-dominated shell because of the significantly different thermal expansion coefficients of the three metals. Beyond the overall melting, all nanoparticles transform into a trimetallic mixing alloy coated by an Au-dominated surface. This work provides a fundamental perspective on the thermodynamic behaviors of trimetallic, even multimetallic, nanoparticles at the atomistic level, indicating that controlling the alloy composition is an effective strategy to realize tunable thermal stability of metallic nanocatalysts.

Resolving structural transitions in spherical dust clusters

Finite systems in confining potentials are known to undergo structural transitions similar to phase transitions. However, these systems are inhomogeneous, and their "melting" point may depend on the position in the trap and vary with the particle number. Focusing on three-dimensional Coulomb systems in a harmonic trap a rich physics is revealed: in addition to radial melting we demonstrate the existence of intrashell disordering and intershell angular melting. Our analysis takes advantage of a novel melting criterion that is based on the spatial two- and three-particle distribution functions and the associated reduced entropy which can be directly measured in complex plasma experiments.

Thermal and shape stability of high-index-faceted rhodium nanoparticles: a molecular dynamics investigation

Atomistic simulations are used to investigate the thermodynamic and shape stability of tetrahexahedral Rh nanaoparticles with high-index facets.

Al20+ does melt, albeit above the bulk melting temperature of aluminium

Employing first principles parallel tempering molecular dynamics in the microcanonical ensemble, we report the presence of a clear solid–liquid-like melting transition in Al₂₀⁺ clusters, not found in experiments.

Diverse melting modes and structural collapse of hollow bimetallic core-shell nanoparticles: a perspective from molecular dynamics simulations

Introducing hollow structures into metallic nanoparticles has become a promising route to improve their catalytic performances. A fundamental understanding of thermal stability of these novel nanostructures is of significance for their syntheses and applications. In this article, molecular dynamics simulations have been employed to offer insights into the thermodynamic evolution of hollow bimetallic core-shell nanoparticles. Our investigation reveals that for hollow Pt-core/Au-shell nanoparticle, premelting originates at the exterior surface, and a typical two-stage melting behavior is exhibited, similar to the solid ones. However, since the interior surface provides facilitation for the premelting initiating at the core, the two-stage melting is also observed in hollow Au-core/Pt-shell nanoparticle, remarkably different from the solid one. Furthermore, the collapse of hollow structure is accompanied with the overall melting of the hollow Pt-core/Au-shell nanoparticle while it occurs prior to that of the hollow Au-core/Pt-shell nanoparticle and leads to the formation of a liquid-core/solid-shell structure, although both of them finally transform into a mixing alloy with Au-dominated surface. Additionally, the existence of stacking faults in the hollow Pt-core/Au-shell nanoparticle distinctly lowers its melting point. This study could be of great importance to the design and development of novel nanocatalysts with both high activity and excellent stability.

Computational approaches elucidate the allosteric mechanism of human aromatase inhibition: a novel possible route to Small-molecule regulation of CYP450s activities?

Human aromatase (HA) is a P450 cytochrome (CYP) with an essential role in estrogen biosynthesis. Since more than 70% of breast cancers are positive for estrogenic receptor (ER), the reduction of estrogen physiological concentrations through HA inhibition is one of most important therapeutic strategies against this cancer type. Recently, experimental evidence showed that selected taxmoxifen metabolites, which are typically used as estrogen receptor modulators (SERMs), inhibit HA through an allosteric mechanism. In this work, we present a computational protocol to (i) characterize the structural framework and (ii) define the atomistic details of the determinants for the noncompetitive inhibition mechanism. Our calculations identify two putative binding sites able to efficiently bind all tamoxifen metabolites. Analysis of long-scale molecular dynamics simulations reveal that endoxifen, the most effective noncompetitive inhibitor, induces significant enzyme rigidity by binding in one of the possible peripheral sites. The consequence of this binding event is the suppression of one of the functional enzymatic collective motions associated with breathing of the substrate access channel. Moreover, an internal dynamics-based alignment of HA with six other human cytochromes shows that this collective motion is common to other members of the CYP450 protein family. On this basis, our findings may thus be of help for the development of new (pan)inhibitors for the therapeutic treatment of cancer, targeting and modulating the activity of HA and of estrogen receptor, and may also stimulate the development of new drug design strategies for chemoprevention and chemoprotection via allosteric inhibition of CYP450 proteins.

Nanothermodynamics of large iron clusters by means of a flat histogram Monte Carlo method

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 σ-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 S(ij) 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 T(m) is plotted in terms of the cluster atom number N(at). The standard N(at)(-1/3) linear dependence (Pawlow law) is observed for N(at) >300, allowing an extrapolation up to the bulk metal at 1940 ±50 K. For N(at) <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.

Stationary solutions for the 2+1 nonlinear Schrödinger equation modeling Bose-Einstein condensates in radial potentials

Stationary solutions for the 2+1 cubic nonlinear Schrödinger equation modeling Bose-Einstein condensates (BEC) in a small potential are obtained via a form of perturbation. In particular, perturbations due to small potentials which either confine or repel the BECs are studied, and under arbitrary piecewise continuous potentials, we obtain the general representation for the perturbation theory of radial BEC solutions. Numerical results are also provided for regimes where perturbative results break down (i.e., the large-potential regime). Both repulsive and attractive BECs are considered under this framework. Solutions for many specific confining potentials of physical relevance to experiments on BECs are provided in order to demonstrate the approach. We make several observations regarding the influence of the particular small potentials on the behavior of the BECs.

Thermodynamic and conformational insights into the phase transition of a single flexible homopolymer chain using replica exchange Monte Carlo method

The phase transition of a single flexible homopolymer chain in the limit condition of dilute solution is systematically investigated using a coarse-grained model. Replica exchange Monte Carlo method is used to enhance the performance of the conformation space exploration, and thus detailed investigation of phase behavior of the system can be provided. With the designed potentials, the coil-globule transition and the liquid-solid-like transition are identified, and the transition temperatures are measured with the conformational and thermodynamic analyses. Additionally, by extrapolating the coil-globule transition temperature, T Θ , and the liquid-solid-like transition temperature T(L → S) to the thermodynamic limit, N → ∞, we found no "tri-critical" point in the current model.

Continuous-time random-walk approach to supercooled liquids. I. Different definitions of particle jumps and their consequences

Single-particle trajectories in supercooled liquids display long periods of localization interrupted by "fast moves." This observation suggests a modeling by a continuous-time random walk (CTRW). We perform molecular dynamics simulations of equilibrated short-chain polymer melts near the critical temperature of mode-coupling theory Tc and extract "moves" from the monomer trajectories. We show that not all moves comply with the conditions of a CTRW. Strong forward-backward correlations are found in the supercooled state. A refinement procedure is suggested to exclude these moves from the analysis. We discuss the repercussions of the refinement on the jump-length and waiting-time distributions as well as on characteristic time scales, such as the average waiting time ("exchange time") and the average time for the first move ("persistence time"). The refinement modifies the temperature (T) dependence of these time scales. For instance, the average waiting time changes from an Arrhenius-type to a Vogel-Fulcher-type T dependence. We discuss this observation in the context of the bifurcation of the α process and (Johari) β process found in many glass-forming materials to occur near Tc. Our analysis lays the foundation for a study of the jump-length and waiting-time distributions, their temperature and chain-length dependencies, and the modeling of the monomer dynamics by a CTRW approach in the companion paper [J. Helfferich et al., Phys. Rev. E 89, 042604 (2014)].

Stationary solutions for the 1+1 nonlinear Schrödinger equation modeling attractive Bose-Einstein condensates in small potentials

Stationary solutions for the 1+1 cubic nonlinear Schrödinger equation (NLS) modeling attractive Bose-Einstein condensates (BECs) in a small potential are obtained via a form of nonlinear perturbation. The focus here is on perturbations to the bright soliton solutions due to small potentials which either confine or repel the BECs: under arbitrary piecewise continuous potentials, we obtain the general representation for the perturbation theory of the bright solitons. Importantly, we do not need to assume that the nonlinearity is small, as we perform a sort of nonlinear perturbation by allowing the zeroth-order perturbation term to be governed by a nonlinear equation. This is useful, in that it allows us to consider perturbations of bright solitons of arbitrary size. In some cases, exact solutions can be recovered, and these agree with known results from the literature. Several special cases are considered which involve confining potentials of specific relevance to BECs. We make several observations on the influence of the small potentials on the behavior of the perturbed bright solitons. The results demonstrate the difference between perturbed bright solitons in the attractive NLS and those results found in the repulsive NLS for dark solitons, as discussed by Mallory and Van Gorder, [Phys. Rev. E 88 013205 (2013)]. Extension of these results to more spatial dimensions is mentioned.

Graphene nucleation on a surface-molten copper catalyst: quantum chemical molecular dynamics simulations

Molecular simulations reveal how graphene grows on copper surfaces, and that defects in the graphene structure are continually removed by mobile copper atoms in the surface layer of the catalyst.

Nanothermodynamics of metal nanoparticles

This article presents a perspective on thermodynamic characterization of metal nanoparticles by computational chemistry. Topics emphasized include structural stability, phases, phase changes, and free energy functions of aluminum nanoparticles.

Stationary solutions for the 1+1 nonlinear Schrödinger equation modeling repulsive Bose-Einstein condensates in small potentials

Stationary solutions for the 1+1 cubic nonlinear Schrödinger equation modeling repulsive Bose-Einstein condensates (BEC) in a small potential are obtained through a form of nonlinear perturbation. In particular, for sufficiently small potentials, we determine the perturbation theory of stationary solutions, by use of an expansion in Jacobi elliptic functions. This idea was explored before in order to obtain exact solutions [Bronski, Carr, Deconinck, and Kutz, Phys. Rev. Lett. 86, 1402 (2001)], where the potential itself was fixed to be a Jacobi elliptic function, thereby reducing the nonlinear ODE into an algebraic equation, (which could be easily solved). However, in the present paper, we outline the perturbation method for completely general potentials, assuming only that such potentials are locally small. We do not need to assume that the nonlinearity is small, as we perform a sort of nonlinear perturbation by allowing the zeroth-order perturbation term to be governed by a nonlinear equation. This allows us to consider even poorly behaved potentials, so long as they are bounded locally. We demonstrate the effectiveness of this approach by considering a number of specific potentials: for the simplest potentials, and we recover results from the literature, while for more complicated potentials, our results are new. Dark soliton solutions are constructed explicitly for some cases, and we obtain the known one-soliton tanh-type solution in the simplest setting for the repulsive BEC. Note that we limit our results to the repulsive case; similar results can be obtained for the attractive BEC case.