Pictures from LAMMPS Simulations

The images on this page, submitted by users, are from LAMMPS simulations. They have been rendered with various visualization packages. This page has additional images with accompanying animations.

	hypervelocity impacts in tungsten		phase behavior of diblock copolymers
	polyethylene nano-droplets		ion-exchange membranes
	superionic water ice		hydrogel degradation
	LAMMPS overview paper		flow and arrest in stressed granular materials
	reactive photopolymers		ChemSNAP potentials
	PEGDA hydrogels		lipids immobilizing water into droplets
	thermoset reactions		silver nanoparticles
	colloidal spherocylinder films		thermal degradation of polyethylene
	nanowire surface dislocation reflection		dislocation dynamics
	acid-containing polymer dynamics		density-functional tight-binding
	rigid polymer nanoparticle coatings		BOP potential for carbon
	nanoprojectile impacts		jamming of extremely polydisperse granular materials
	polymeric fullerene film coating		ELBA coarse-grained water
	electrochemical adsorption of OH		nanoparticle assembly and aggregation
	liquid crystal film rupture		polymer shear thinning
	ionomer morphologies		amorphous carbon film growth
	electro-catalytically active gold nanoparticles		bilayer phases
	thermal conductivity of CNTs		free energies via LAMMPS and PLUMED
	capillary filling in CNTs		shock loading of polymer foam
	carbon nanotube fiber design		shock loading of inhomogeneous PBX
	single-point diamond turning		nanoparticle coating structure in the presence of solvent
	two-temperature model for electronic heat conduction		atom-to-continuum coupling with the ATC package
	stress field around dislocations		water interacting with self-assembled monolayers
	coarse-grained self-assembly of lipids and PEG surfactants		spherical polyelectrolyte brushes
	coarse-grained block copolymer generation		polyelectrolyte adsorption and brushes
	stress in metal nanowires with twin boundaries		nanotip indentation of a coated surface
	surface wetting by polymer nanodroplet		shear of Cu bicrystal
	solvated dendritic polymer phase behavior		metal solidification
	lipid membrane self-assembly and fusion		tensile pull on adhesive polymer chains
	crazing of entangled polymer chains		stress in metal nanowires
	shear of large single-crystal metals

All of the images below are shown in small size. Click on the image to view a larger version.

Hypervelocity impacts in tungsten

This is work by Alberto Fraile, Prashant Dwivedi, Giovanni Bonny and Tomas Polcar at the Czech Technical University (Prague), Bangor University (UK), Nuclear Materials Science Institute (Belgium).

Atomistic mechanisms of damage initiation during high velocity (up to 9 km/s, kinetic energies up to 200 keV) impacts of W projectiles on a W surface were investigated using parallel molecular-dynamics simulations involving large samples (up to 40 million atoms).

Various aspects of the impacts, where the projectile and part of the target material undergo massive plastic deformation, breakup, melting, and vaporization, were analyzed. Different stages of the penetration process have been identified through a detailed examination of implantation, crater size and volume, sputtered atoms, and dislocations created by the impacts. The crater volume increases linearly with the kinetic energy for a given impactor; and the total dislocation length (TDL) increases with the kinetic energy but depends on the size of the impactor. We found that the TDL does not depend on the used interatomic potential. The results are rationalized based on the physical properties of bcc W.

The image on the journal cover shows a representative impact. The additional figure shows 2 nm size W particle impacting a W single-crystal target with v = 2 km/s , at times 5, 20 and 100 ps after the impact. The top row images are color coded following CNA analysis; blue for bcc coordination, green for fcc coordination, red for hcp coordination, and gray for unidentified structure, i.e. liquid/vaporized in this case. The bottom row images are the same systems, showing only dislocations in the target. The colours indicate the dislocation type: green curves are ½<111> dislocation segments while pink ones correspond to <100> dislocation segments. Blue surfaces indicate the sputtered atoms and crater formation.

Analysis of hypervelocity impacts: the tungsten case, A. Fraile, P. Dwivedi, G. Bonny and T. Polcar, Nuclear Fusion, 62, 026034 (2022). (abstract)

Phase behavior of diblock copolymers

This work is by Andrew Wijesekera, Daniel L. Vigil, and Ting Ge from The University of South Carolina and Sandia National Laboratories where MD simulations were used to compare the differences in self-assembly between linear and ring diblock polymers.

The image on the journal cover shows a single diblock ring polymer surrounded by other chains in their primitive path states. The additional figure illustrates the differences in self-assembly dynamics of ring and linear diblock polymers.

Molecular Simulations Revealing Effects of Non-concatenated Ring Topology on Phase Behavior of Symmetric Diblock Copolymers, A. Wijesekera, D. L. Vigil, T. Ge, Macromolecules, 57, 5092-5104 (2024). DOI: https://doi.org/10.1021/acs.macromol.3c02473 (abstract)

Polyethylene nano-droplets

This work is by Hasan Zerze from The University of Houston and uses MD simulations to study glass transition and nucleation and growth of crystallization in polyethylene nanodroplets with different sizes.

The image on the journal cover illustrates a pure polyethylene droplet before and sometime after it is quenched to a temperature below melting temperature. The second image shows the evolution of the crystallization process using snapshots taken at different times, as well as a plot of the time dependent crystallinity for different droplet sizes. This study found that the volumetric crystallization rate enhances as the droplet size decreases.

Nucleation and growth of crystals inside polyethylene nano-droplets, H. Zerze, J Chem Phys, 157, 154901 (2022). DOI: https://doi.org/10.1063/5.0105466 (abstract)

Exponential water uptake by ionomer membranes

This work is by Adam Barnett, John Karnes, Jibao Lu, Dale Major, James Oakdale, Kyle Grew, Joshua McClure, and Valeria Molinero from The University of Utah, Lawrence Livermore National Laboratory, and the Army Research Laboratory and uses hybrid MD/GCMC simulations to calculate the hydration of model anion exchange membranes as a function of water activity and ionomer design.

The cover is an artistic rendering of GCMC insertion/deletion of water molecules with the simulation cell located in the center of the image. These simulations capture the onset of exponential water uptake at high water activity and quantify the effect of ionomer design on water uptake. The other figure quantifies the effect of higher ion exchange capacity (the number of charged sites per mass of ionomer) on water uptake.

Exponential Water Uptake in Ionomer Membranes Results from Polymer Plasticization, A. Barnett, J. J. Karnes, J. Lu, D. R. Major Jr., J. S. Oakdale, K. N. Grew, J. P. McClure, V. Molinero, Macromolecules, 55, 6762–6774 (2022). DOI: https://doi.org/10.1021/acs.macromol.2c01042 (abstract)

Plastic deformation of superionic water ices

This is work by Filipe Matusalem, Jéssica Santos Rego and Maurice de Koning, from Universidade Estadual de Campinas (UNICAMP), Brazil, modeling the plastic flow behavior of superionic water ice.

The cover displays an image of superionic ice, in which oxygen ions (red spheres) occupy a regular crystal lattice, whereas protons (white spheres) flow through it as a liquid. Based on ideal shear-strength estimates from DFT calculations (shown as stress-strain curves), the study constructs a machine-learning interatomic potential to determine dislocation-glide velocities as a function of the applied shear stress (shown in velocity vs stress curves) using the LAMMPS code. The corresponding values for the effective viscosity indicate that these phases may flow much more easily than anticipated, possibly affecting the interior dynamics of the ice giants, Neptune and Uranus.

Plastic deformation of superionic water ices, F. Matusalem, J. Santos Rego, M. de Koning, PNAS, 119 (45), e2203397119 (2022). DOI: https://doi.org/10.1073/pnas.2203397119 (abstract)

Controlled degradation of hydrogels

This is work by Vaibhav Palkar and Olga Kuksenok at Clemson University to study the controlled degradation and erosion of polymer networks at the mesoscale. This work demonstrates an example usage of the pair srp/react command in LAMMPS.

The second image is a schematic of a part of the polymer network architecture prior to degradation. The third image has snapshots of degradation of an equilibrated hydrogel film. The last image shows polymer precursors stuck inside a degrading hydrogel film.

Controlling Degradation and Erosion of Polymer Networks: Insights from Mesoscale Modeling, V. Palkar and O. Kuksenok, Journal of Physical Chemistry B, 126, 336-346 (2022). (abstract)

LAMMPS overview paper

This is work by the LAMMPS development team and many collaborators at a variety of institutions to write a new LAMMPS overview paper, which appeared in Comp Phys Comm in 2022.

The journal cover illustrates four simulation models run with LAMMPS across a range of length and time scales. The caption and credits for the images is on the 2nd cover image. The other 3 figures are from the paper and illustrate the kinds of problems which the code can effectively load-balance for good parallel performance. The first is a virus-like particle budding via interaction with a cell membrane (courtesy of John Grime and Gregory Voth, U Chicago). The second is a hollow metal strut as an example of a nanoengineered material (courtesy of Alexander Stukowski, Technische Universität Darmstadt). The third is a coarse-grained model of the catastrophic depolymerization of alpha/beta-tubulin (courtesy of Mark Stevens, Sandia).

LAMMPS - A flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales, A. P. Thompson, H. M. Aktulga, R. Berger, D. S. Bolintineanu, W. M. Brown, P. S. Crozier, P. J. in 't Veld, A. Kohlmeyer, S. G. Moore, T. D. Nguyen, R. Shan, M. J. Stevens, J. Tranchida, C. Trott, S. J. Plimpton, Comp Phys Comm, 271, 108171 (2022). DOI:https://doi.org/10.1016/j.cpc.2021.108171 (abstract)

Flow and arrest in stressed granular materials

This work by Ishan Srivastava and collaborators at Sandia National Laboratories describes states of steady flow and shear arrest in frictional granular materials that are subjected to external stresses, and identifies the critical boundary that bifurcates these steady states. This work used the GRANULAR package in LAMMPS.

The journal cover illustrates the evolution of load-bearing force chain network as a granular material transitions from a sparse force network observed during steady flow for a large external stress (left side of cover) to a dense, percolating force network observed during shear arrest for a small external stress (right side). The diagram illustrates the simulation protocol.

Flow and arrest in stressed granular materials, I. Srivastava, L. E. Silvert, J. B. Lechman, G. S. Grest, Soft Matter, 18, 735 (2022). (abstract)

Reactive Photopolymers

This is work by John Karnes and collaborators at Lawrence Livermore National Laboratory to study the reactive dynamics (cross-linking) of various model photopolymers and their resulting network structures. This work used the fix bond/react command in LAMMPS.

The journal cover shows the time evolution of the polymererization of the model acrylate monomer PEGDA. The first image is for a simple model acrylate monomer PETA. The second image has details for 3 different models: PETA, HDDA, and PEGDA. The bottom row are images for PEGDA at different cross-linking fractions.

On the Network Topology of Cross-Linked Acrylate Photopolymers: A Molecular Dynamics Case Study, J. J. Karnes, T. H. Weisgraber, J. S. Oakdale, M. Mettry, M. Shusteff, and J. Biener, Journal of Physical Chemistry B, 124, 9204-9215 (2020). (abstract)

ChemSNAP potentials

This is work by Mary Alice Cusentino, Mitch Wood, and Aidan Thompson (Sandia) which formulates an extension to the Spectral Neighbor Analysis Potential (SNAP) descriptors to develop improved SNAP interatomic potentials for chemically complex systems (ChemSNAP).

The journal cover depicts the di-antisite defect structure in crystalline indium phosphide. Spheres represent locations of indium (purple) and phosphorous (white) atoms, respectively. The fingerprints represent the new explicit multi-element EME-SNAP descriptors that characterize local arrangements of indium and phosphorous atoms. These "fingerprints" are used to accurately estimate the potential energy surface of indium phosphide, including the relaxed formation energies for the important defect structures. The other image is a diagram of the workflow which couples several tools to fit the original SNAP and new ChemSNAP machine learned potentials. It uses LAMMPS and FitSnap.py and the DAKOTA optimization toolkit.

Explicit Multielement Extension of the Spectral Neighbor Analysis Potential for Chemically Complex Systems, M. A. Cusentino, M. A. Wood, and A. P. Thompson, Journal of Physical Chemistry A, 124, 5456-5464 (2020). (abstract)

PEGDA hdrogels

This is work by Ke Luo and collaborators in Doug Spearot's group (U Florida) to simulate the high strain rate behavior of PEGDA hydrogels in the presence of loop defects as a function of cross-link density and functionality.

The cover image shows a cross-linked structure. The plot shows actve ratios in x and y as a function of strain for different functionalities. Snapshots of structure at zero and 114% uniaxial strain are shown for ideal and functionality=3 systems.

Effect of Loop Defects on the High Strain Rate Behavior of PEGDA Hydrogels: A Molecular Dynamics Study, K. Luo, C. Wangari, G. Subhash, and D. E. Spearot, J Phys Chem B, 124, 2029−2039 (2020). (abstract)

Lipids immobilizing water into droplets

This work is by Giacomo Fiorin (Temple) and coworkers using coarse-grained simulations with LAMMPS to show that the lipids in the outer skin layer immobilize excess water into nanometer-size droplets.

The figure on the cover of the Biophysical Journal shows the organization of the water droplets (white density), with a section of the corresponding lipid structure overlaid (bottom right corner). The two additional figures show a self-assembled multi-lamellar structure (128,000 lipid molecules), and a small section of a single lamella with one water droplet enclosed in it. All figures were rendered with VMD.

Coexistence of lipid phases stabilizes interstitial water in the outer layer of mammalian skin, C. M. MacDermaid, K. W. Hall, R. H. DeVane, M. L. Klein, and G. Fiorin, Biophysical Journal 118, 1588–1601 (2020). DOI: https://doi.org/10.1016/j.bpj.2020.01.044 (abstract)

Thermoset reactions and characterization

This is work by Felix Schwab (Karlsruhe Institute of Technology, Germany) and Colin Denniston (U of Western Ontario) to curing reactions (e.g. cross-linking) in polyester polyurethane hybrid resins. Reactions are modeled in classical MD based on atom distances and Arrhenius law probabilities.

Reaction and characterisation of a two-stage thermoset using molecular dynamics, F. K. Schwab and C. Denniston, Polymer Chemistry, 10, 4413-4427 (2019). (abstract)

Silver nanoparticles

This is work by Edison da Silva and collaborators (Institute of Physics, UNICAMP, Campinas, SP, Brazil) to study the sintering processes of Ag nanoparticles. Silver tungstate irradiated by an electron beam generates metallic silver filaments, with concomitant formation of silver nanoparticles. The effect of the electron beam produces surface plasma resonances(SPR) to the nanoparticles. These silver nanoparticles therefore, become interacting electric nano-dipoles that undergo sintering processes. Connecting theory, experiment, and long-time atomistic molecular dynamics simulations provides a deep insight into the observed experimental phenomena.

How Crystallization Affects the Oriented Attachment of Silver Nanocrystals, G. M. Faccin, Z. S. Pereira and E. Z. da Silva, J Phys Chem C, 125, 6812-6820 (2021). (abstract)

Connecting Theory with Experiment to Understand the Sintering Processes of Ag Nanoparticles, E. da Silva, G. M. Faccin, T. R. Machado, N. G. Macedo, M. de Assis, S. Maya-Johnson, J. C. Sczancoski, J. Andres, E. Longo, and M. A. San-Miguel, J Phys Chem C, 123, 11310-11318 (2019). (abstract)

Colloidal spherocylinder films

This is work by T. Li and collaborators (University of Edinburgh, United Kingdom) to study films made of colloidal spherocylinders, to observe interesting dynamic and self-assembly events.

Colloidal Spherocylinders at an Interface: Flipper Dynamics and Bilayer Formation, T. Li, G. Brandani, D. Marenduzzo, and P. S. Clegg, Phys Rev Letters, 119, 018001 (2017). (abstract), doi.org/10.1103/PhysRevLett.119.018001

High-rate thermal degradation of polyethylene

This work by Matt Lane and Nathan Moore (Sandia), aims to understand the high-rate thermal decomposition of organics by simulation of conditions comparable to recent Z machine X-ray ablation experiments. They determined the rate-dependent chemical processes were important to understanding the physics of polymer degradation. They also developed a kinetic model of these processes for polyethylene which is being incorporated into future continuum-scale calculations.

The images show the molecular states of polyethylene at 300, 2100 and ~3000 K before/during/after an extremely high-rate temperature ramp from 10^10 to 10^14 K/s, such as expected in laser-driven and X-ray ablation experiments. Carbon atoms are color-coded by the number of C–C bonds they share. Orange, for one bond, indicates a terminal carbon. Blue, for two bonds, indicates a backbone or cyclic carbon. Green, for three or more C–C bonds, indicates a branch point, or a condensed carbon phase. Hydrogen atoms are white or, if they form molecular hydrogen H2, yellow.

Nanowire surface dislocation reflection

This is work by G. Sainath and collaborators (Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam, India).

MD was used to model the multiple reflections of partial dislocations from nanowire surfaces. The continuous repetition of this process along the length of nanowire results in the formation of stacking faults with triangular wave shape. This reflection of dislocations has been observed in Cu nanowires with size less than 2.17 nm.

Size dependent deformation behaviour and dislocation mechanisms in <100> Cu nanowires, G. Sainath , P. Rohith and B. K. Choudhary, Philosophical Magazine, 97, 2632-2657 (2017). (abstract), doi.org/10.1080/14786435.2017.1347300

Dislocation dynamics

This is work by Luis Zepeda-Ruiz, Tomas Oppelstrup, and Vasily Bulatov at LLNL, and Alex Stukowski (Technische Universitat Darmstadt), to model dislocation dynamics in bcc tantalum.

The cover image is from Alex Stukowski, with the accompanying text from the journal. The stress-strain curves are also a figure from the paper.

The cover shows an intricate network of lattice defects — dislocation lines — whose motion makes metal tantalum flow under compression. Fully dynamic atomistic simulations of plastic deformation in metals are extremely demanding computationally and usually involve mesoscale approximations. In this issue, Vasily Bulatov and his colleagues present fully dynamic atomic-level simulations of metal plasticity that feature up to 268 million atoms, each such simulation generating around 2 exabytes (1 exabyte = 1018 bytes) of data. Using their model, the authors probe how body-centred-cubic metal tantalum responds to ultrahigh-strain-rate deformation. They find that on reaching certain limiting conditions, dislocations can no longer relieve mechanical loads and twinning — the sudden reorientation of the crystal lattice — takes over. They also find that below such critical conditions, flow stress and dislocation density achieve a steady state in which the metal can be kneaded indefinitely like a piece of dough.

Probing the limits of metal plasticity with molecular dynamics simulations, L. A. Zepeda-Ruiz, A. Stukowski, T. Oppelstrup, and V. V. Bulatov, Nature, 550, 492 (2017). (abstract), doi:10.1038/nature23472.

Acid-containing polymer chain dynamics

This is work by Amalie Frischknecht (Sandia), Karen Winey (U Penn), and coworkers to understand chain dynamics in precise acid-containing polymers.

The journal cover page shows the comparison of quasi-elastic neutron scattering (QENS) and atomistic molecular dynamics simulations for poly(ethylene-co-acrylic acid) polymers that have acid groups spaced every 21 carbons along the polyethylene backbone. The simulation snapshots show the movement of acid groups from one aggregate to another over approximately 1 ns.

Heterogeneous Chain Dynamics and Aggregate Lifetimes in Precise Acid-Containing Polyethylenes: Experiments and Simulations, L. R. Middleton, J. D. Tarver, J. Cordaro, M. Tyagi, C. L. Soles, A. L. Frischknecht, and K. I. Winey, Macromolecules 49, 9176-9185 (2016). (abstract), http://pubs.acs.org/doi/abs/10.1021/acs.macromol.6b01918

Density-functional tight-binding models

This is work by Nir Goldman (LLNL) and collaborators using a modified version of the DFTB+ code which can be hooked to LAMMPS through its library interface to perform quantum MD simulations. DFTB+ computes quantum density-functional forces via the semi-empirical tight-binding approximation; LAMMPS does the timestepping.

They have used this coupled framework to study a variety of interesting systems, as shown in these journal cover images. The first is synthesis of carbon fibers from graphite which has been liquified and evaporates due to a laser pulse. The second is a model of a laser pulse inducing a shock which triggers exothermic chemistry in hydrogen peroxide. The final example models the creation of prebiotic organic molecules via shock compression of icy materials, analogous to what may occur when a comet strikes the surface of the Earth.

Carbyne fiber synthesis in evaporating metallic liquid carbon, C. Cannella and N. Goldman, J Phys Chem C, 21605, 119 (2015). (abstract)

Control of reactivity via ultrafast compression rates, M. R. Armstrong, J. M. Zaug, N. Goldman, I-F. W. Kuo, J. C. Crowhurst, W. M. Howard, J. A. Carter, M. Kashgarian, J. M. Chesser, T. W. Barbee, and S. Bastea, J Phys Chem A, 117, 13051 (2013). (abstract)

Prebiotic Chemistry Within a Simple Impacting Icy Mixture, N. Goldman and I. Tamblyn, J Phys Chem A, 117, 5124 (2013). (abstract)

Rigid polymer nanoparticle coatings

This is work by Sabina Maskey & Dvora Perahia of Clemson University and Matt Lane (jlane at sandia.gov) & Gary Grest at Sandia, which describes the unique morphologies possible when nanoparticles are grafted with rigid luminescent polymers like dialkyl poly(p-phenylene ethynylene)s (PPEs) and placed in poor solvents. The macromolecules assemble into bundles, exhibiting a distinctively different interfacial structure than that formed by grafted flexible and semiflexible polymers.

The images show the rigid nanoparticle (yellow) and grafted polymers (green) in different morphologies from compact to extended.

Structure of Rigid Polymers Confined to Nanoparticles: Molecular Dynamics Simulations Insight, S. Maskey, J. M. D. Lane, D. Perahia, and G. S. Grest, Langmuir, 32, 2102–2109 (2016). (http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.5b04568) (abstract)

BOP potential for carbon

This is work by Xiaowang Zhou (xzhou at sandia.gov), Don Ward, and Michael Foster at Sandia to develop and test a new bond-order potential (BOP) suitable for carbon in many of its forms (diamond, graphite, graphene, nanotubes). The potential is available in LAMMPS via the pair_style bop command.

An Analytical Bond-Order Potential for Carbon, X. W. Zhou, D. K. Ward, and M. E. Foster, J Comp Chem, 36, 1719-1786 (2015). (abstract)

Nanoprojectile impacts

This is work by Eduardo Bringa (ebringa at yahoo.com) at Universidad Nacional de Cuyo (Argentina) and collaborators to model the impact of nanoparticles up to 55 nm in diameter hitting substrate targets with up to 10 billion particles. The difference in cratering mechanisms was found to change for small vs large particles with the latter transitioning to plastic flow mechanisms characterisitic of microparticle experiments. The large simulations were performed with LAMMPS.

Why Nanoprojectiles Work Differently than Macroimpactors: The Role of Plastic Flow, C. Anders, E. M. Bringa, G. Ziegenhain, G. A. Graham, J. F. Hansen, N. Park, N. E. Teslich, and H. M. Urbassek, Phys Rev Lett 108, 027601 (2012). (abstract)

Jamming and deformation of extremely polydisperse granular materials

This is work by Agnieszka Herman (oceagah at ug.edu.pl) at the Institute of Oceanography, University of Gdansk (Poland) to model the response of two-dimensional, extremely poly-disperse granular materials (with power law grain size distribution) to various types of deformation (convergence, shear, etc.). The work concentrates on the role of polydispersity in shaping the behavior and properties of the modeled system. The model is applied to analyze the deformation of sea ice composed of individual, interacting ice floes in response to wind and/or ocean currents.

The first two figures show snapshots of contact networks in the modeled system subject to shear deformation. The last two show the correlation coefficient between velocity anomalies of a selected grain (dark brown) and all other grains. In the first and third figures, an unjammed state are shown; in the second and fourth a jammed state is shown, for comparison.

Shear-jamming in two-dimensional granular materials with power-law grain-size distribution, A. Herman, Entropy, 15, 4802-4821 (2013). (abstract)

Numerical modeling of force and contact networks in fragmented sea ice, A. Herman, Annals Glaciology, 54, 114-120 (2013). (abstract)

Polymeric fullerene film coated on Si

This is work by Minwoong Joe (mjoe122 at gmail.com), Young-Kyu Han, Kwang-Ryeol Lee, Hiroshi Mizuseki, and Seungchul Kim (sckim at kist.re.kr) at KIST to model polymeric fullerene coatings on Si electrodes in Lithium ion batteries, which leads to durability of the battery performance, using the Tersoff potential with parameters suggested by Erhart and Albe.

The first figure shows a graphical representation of the polymerized fullerene film grown at E = 180 eV. Each C60 molecule is a different color, but the same color is used when they are connected by carbon atoms for which the coordination number CN = 3. Black balls are carbon atoms with CN = 2 and 4.

The second figure shows the superior mechanical flexibility of the polymerized fullerene coating on Si. Even under conditions of substrate Si rupture by huge compressive (b) and tensile strain (c), the polymeric fullerene film does not undergo mechanical failure, implying superior flexibility to adapt to the volume change of the Si anode. The atoms color is encoded using the CN: burly wood for 2; forest green for 3; gray for 4; and red for 5.

An ideal polymeric C60 coating on a Si electrode for durable Li-ion batteries, M. Joe, Y.-K. Han, K.-R. Lee, H. Mizuseki, S. Kim, Carbon, 77, 1140 (2014). (abstract)

Mixing of atomistic solutes and coarse-grained water

This is work by Mario Orsi (m.orsi at qmul.ac.uk) and coworkers at QMUL. The ELBA coarse-grained water is a single-site "Lennard-Jones plus point dipole model" which can capture many fundamental properties of bulk liquid water and the water-vapor interface. The model can also be used to hydrate atomistic solutes, including small organic molecules and proteins; importantly, the ELBA water force field is shown to be directly compatible with common atomistic force fields.

The images below show a sketch of the coarse-graining strategy (left), a vapor-liquid phase diagram (left-center), and two snapshots of simulations of atomistic proteins hydrated by ELBA water (right-center and right).

Direct Mixing of Atomistic Solutes and Coarse-Grained Water, M. Orsi, W. Ding, M. Palaiokostas, Journal of Chemical Theory and Computation, 10, 4684-4693 (2014); http://pubs.acs.org/doi/abs/10.1021/ct500065k. (abstract)

Comparative assessment of the ELBA coarse-grained model for water, M. Orsi, Molecular Physics,112, 1566-1576 (2014); http://dx.doi.org/10.1080/00268976.2013.844373. (abstract)

Electrochemical Adsorption of OH

This is work by Wolfgang Schmickler (wolfgang.schmickler at uni-ulm.de) at the University of Ulm and coworkers to investigate the adsorption of the hydroxyl ion on a Pt(111) surface. To perform this study a combination of DFT with molecular dynamics was used and the results suggest that OH can be adsorbed either as a metastable, physisorbed ion or as a chemisorbed radical.

The picture on the left shows a snapshot from a molecular dynamics simulation; the OH can be seen on the left with a blue oxygen and a green hydrogen atom. The middle picture presents the Volumetric charge density difference for the OH adsorption on a Pt(111) surface; blue indicates electron depletion and red stands for electron excess.

Electrochemical Adsorption of OH on Pt(111) in Alkaline Solutions: Combining DFT and Molecular Dynamics, L. M. C. Pinto, P. Quaino, M. D. Arce, E. Santos, and W. Schmickler, Chem Phys Chem, 15, 2003-2009 (2014); http://dx.doi.org/10.1002/cphc.201400051. (abstract)

Nanoparticle assembly and aggregation

This is work by Matt Lane (jlane at sandia.gov) and Gary Grest at Sandia to model the self-assembly and aggregation of coated nanoparticles at a water-vapor interface. The terminal groups of a nanoparticle coating can dramatically alter the coating shape at the interface, producing very different assembly morphologies

The left and middle images show side and top views of nanoparticles on the surface of a water layer. The yellow spheres are the gold nanoparticles, the alkane chains (coatings) are in blue/white, and the water in red/white. Some chains are terminated with COOH (polar), others with CH3 (non-polar), which determines their affinity for the surrounding water.

Assembly of responsive-shape coated nanoparticles at water surfaces, J. M. D. Lane and G. S. Grest, Nanoscale, 6, 5132-5137 (2014). (abstract)

Liquid crystal film rupture

This is work by Trung Dac Nguyen (nguyentd at ornl.gov), Jan-Michael Carrillo, and Mike Brown at ORNL to model liquid crystal thin films and investigate their stability. These were large-scale simulations of up to 26 million ellipsoidal particles, each representing a LC mesogen, run using the GPU-accelerated GayBerne potential developed by Mike Brown, on the Titan machine at ORNL.

The left image shows a hole formed in a nematic film where the LC mesogens are colored by their distance to the substrate. The image in the middle shows the top-view of a hole in a nematic film where the LC mesogens are colored by their alignment with their neighbors. The right image shows surface undulations in a nematic film where pink corresponds to thick regions and green to thinner regions.

Rupture mechanism of liquid crystal thin films realized by large-scale molecular simulations, T. D. Nguyen, J-M Y. Carrillo, M. A. Matheson, and W. M. Brown, Nanoscale, 6, 3083 (2014). (abstract)

Polymer shear thinning

"This is work by KR Prathyusha (prathyushakr at gmail.com) and collaborators at the Indian Institute of Technology Madras (India) and the University of Memphis to model short polymer segemnts which bind together to form networks, as a model for "living" polymers. When sheared, the systems self-assemble into various morphologies.

The cover page shows a columnar phase, formed when the system is sheared. The right figure shows various shear-induced phases which appear as the interaction parameters are varied.

Shear-thinning and isotropic-lamellar-columnar transition in a model for living polymers, K. R. Prathyusha, A. P. Deshpande, M. Laradji, and P. B. S. Kumar, Soft Matter, 9, 9983 (2013). (abstract)

Ionomer morphologies

This is work by Dan Bolinteneau, Mark Stevens, and Amalie Frischknecht (alfrisc at sandia.gov) at Sandia to model the structure of ionic aggregates in ionomers, which are polymers with both neutral and ionized segments.

The left figure shows representative snapshots of ionic aggregates in simulations of poly(ethylene-co-acrylic acid) ionomer melts with 10%, 43% and 100% Li neutralization (left to right). Only H, Li and O atoms are shown and all atoms in the same aggregate have the same color. The lower right image is a zoom of the 43% Li system showing the interaggregate spacing that yields the ionomer peak in scattering data. The individual aggregate shows an alternating Li (yellow) and O (red) motif, with some hydrogen bonding (H in white). In contrast to the traditional view of spherical ionic aggregates, the schematic in the lower right depicts the string-like aggregates observed in simulations. The right figure gives more details of the structures as a function of neutralization percentage.

Atomistic Simulations Predict a Surprising Variety of Morphologies in Precise Ionomers, D. S. Bolintineanu, M. J. Stevens, and A. L. Frischknecht, ACS Macro Letters, 2, 206-210 (2013). (abstract) (paper)

Amorphous carbon film growth

This is work by Minwoong Joe (mjoe122 at gmail.com), Myoung-Woon Moon, Jungsoo Oh, Kyu-Hwan Lee, and Kwang-Ryeol Lee (krlee at kist.re.kr) at KIST to model impact-induced surface instabilities in amorphous carbon (a-C) film growth, which leads to surface roughening under grazing impacts of energetic C atoms, using the reactive empirical bond order (REBO) potential.

The first figure shows cross-sectional snapshots of the growing film, depending on incidence angles.

The second figure shows a flying view of the surface after 36,000 C impacts on a larger substrate (approximately 9x larger).

The third figure shows a comparison of the cross-sections of a-C film under normal (a) and grazing incidence (b). The color of the deposited atoms encodes the order of atom entry; the later atoms range from blue to red, and the original substrate is grey. This indicates a shadowing effect at work even under 0.2 nm rms surface roughness.

Molecular dynamics simulation study of the growth of a rough amorphous carbon film by the grazing incidence of energetic carbon atoms, M. Joe, M.-W. Moon, J. Oh, K.-H. Lee, K.-R. Lee, Carbon, 50, 404 (2012). (abstract)

Electro-catalytically active gold nanoparticles

This is work by Yang-Hee Lee, Ji-Hoon Jang, Juyeong Kim, and Young-Uk Kwon (ywkwon at skku.edu) at Sungkyunkwan University, Gunn Kim (kimgunn at gmail.com, for DFT calculation) at Sejong University, and Minwoong Joe and Kwang-Ryeol Lee (mjoe122 at gmail.com, for MD simulation) at KIST to model enhancement of electrocatalytic activity of gold nanoparticles, produced by sonochemical treatment, using the embedded atom potential.

The cover page schematically shows supercooled molten gold nanoparticles by sonochemical treatment, which can have enhanced electrocatalytic activity for hydrogen oxidation reaction.

The 2nd figure shows coordination numbers of gold atoms of calculated structures of gold NPs equilibrated at 0 K (a), 300 K (b), 500 K (c), 700 K (d), 900 K (e), and 1100 K (f).

Enhancement of Electrocatalytic Activity of Gold Nanoparticles by Sonochemical Treatment, Y.-H. Lee, G. Kim, M. Joe, J.-H. Jang, J. Kim, K.-R. Lee, Y.-U. Kwon, Chem Comm, 46, 5656 (2010). (abstract)

Bilayer phases

This is work by Mario Orsi (orsimario at gmail.com) at U Southampton. LAMMPS was used together with the ELBA-LAMMPS toolkit to simulate simple models of biological membranes. Notably, depth-dependent lateral pressure and electrical potential profiles were computed for mixed PC/PE bilayers at different relative compositions.

The images below show a sketch of the molecular models (left), a series of snapshots from a self-assembly simulation of a lamellar structure (center), and a series of snapshots from a simulation of spontaneous phase transition from lamellar to inverse hexagonal (right).

Physical properties of mixed bilayers containing lamellar and nonlamellar lipids: insights from coarse-grain molecular dynamics simulations, M. Orsi and J. W. Essex, Faraday Discussions, DOI: 10.1039/C2FD20110K. (abstract) Link to paper

Thermal conductivity of CNTs

This is work by Ajing Cao (a-cao at northwestern.edu) and Jianmin Qu at Northwestern University to use LAMMPS to model the thermal conductivity of single-walled carbon nanotubes. Specifically, they found that the size-dependent thermal conductivity of single-walled carbon nanotubes can be described by κ(L,d) ~ κg(L)(1−e−0.185d/a0), where L is the tube length, d is the diameter, a0 = 2.46 Å is the graphene lattice constant, and κg(L) proportional to Lα is the thermal conductivity of a graphene of length L.

The plots on the cover show the spectral energy density (SED) of single-walled CNTs. The top three figures are for the (5, 5) tube, and the bottom three are for the (40, 40) tube. Both tubes are 100 nm-long SWCNT at T = 300 K. The color indicates the magnitude of the SED at a given point (k,w). From the left to the right are the longitudinal modes Ez, twist modes Eu, and radial breathing modes Er, respectively.

Size dependent thermal conductivity of single-walled carbon nanotubes, A. Cao and J. Qu, J Appl Phys, 112, 013503 (2012); http://dx.doi.org/10.1063/1.4730908 (abstract)

Free energies via LAMMPS and PLUMED

This is work by Andrew Stack (stackag at ornl.gov) at ORNL and collaborators to use LAMMPS in conjunction with the PLUMED free energy package to model growth and dissolution on mineral surfaces via metadynamics. Specifically they looked at barium ions attaching to and detaching from a barite surface. The detachment dynamics and associated free energy diagram are shown in the following figures.

Accurate Rates of the Complex Mechanisms for Growth and Dissolution of Minerals Using a Combination of Rare-Event Theories, A. G. Stack, P. Raiteri, and J. D. Gale, JACS, 134, 11–14 (2012). (abstract)

Capillary filling in CNTs

This is work by Laurent Joly (ljoly.ulyon at gmail.com) of U Lyon on studies of capillary filling on nanopores such as CNTs, examining the effects of liquid viscosity, the friction coefficient between the liquid and wall, and viscous dissipation at the pore entrance.

Capillary filling with giant liquid/solid slip: Dynamics of water uptake by carbon nanotubes, L. Joly, THE JOURNAL OF CHEMICAL PHYSICS, 135, 214705 (2011). (abstract)

Shock loading of polymer foam

This is work by Matt Lane (jlane at sandia.gov) and Aidan Thompson (athomps at sandia.gov) at Sandia to model the shock compression of all-atom polymer foams, using the ReaxFF potential to allow for dissociation of the polymer bonds. Their model captures the Hugoniot response of the foam in good agreement with experiment and DFT calculations.

The images show a PMP system with 1.44M atoms, with a lattice of spherical voids. The initial sample is at the left; the shocked sample is in the middle, which shows jetting of polymer fragments into the voids. These images were used on the cover of a special issue of the MRS Bulletin (May 2012) devoted to many-body potentials like ReaxFF.

Shock compression of dense polymer and foam systems using molecular dynamics and DFT, J. M. D. Lane, G. S. Grest, A. P. Thompson, K. R. Cochrane, M. P. Desjarlais, and T. R. Mattsson, in M. Elert et. al., editor, AIP Conference Proceedings, Shock Compression of Condensed Matter 2011, 1426, 1401 (2011). (abstract)

Computational Aspects of Many-body Potentials", MRS Bulletin, S. J. Plimpton and A. P. Thompson, 37, 513-521 (2012). (abstract)

Carbon nanotube fiber design

This is work by Charles Cornwell (Charles.F.Cornwell at usace.army.mil) and Charles Welch at the US Army ERDC to model the tensile response of bundles of carbon nanotubes containing 1.2M atoms, with additional cross-linking bonds between individual tubes, using the AIREBO force field. They found the fibers had strengths up to 60 GPa, which is about 30x higher than that of high-strength steel.

The first figure shows the bundle geometry. The second shows the rupture of the bundle at high tensile strain.

Very-high-strength (60-GPa) carbon nanotube fiber design based on molecular dynamics simulations, Charles F. Cornwell and Charles R. Welch, J Chem Phys, 134, 204708 (2011). (abstract)

Shock loading of inhomogeneous PBX

This is work by Sergey Zybin (zybin at wag.caltech.edu) and collaborators at Caltech to model shock-induced instabilities in explosive materials which have heterogeneous features, such as defects or interfaces, using the ReaxFF force field.

The figure shows shock loading of PBX in a 3.6M atom model with a saw-tooth interface between RDX and its polymer binder. The color represents slip which is highest at the interface.

Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials, Qi An, Sergey V. Zybin, William A. Goddard III, Andres Jaramillo-Botero, Mario Blanco, and Sheng-Nian Luo, Phys Rev B, 84, 220101 (2011). (abstract)

Single point diamond turning

This is work by Saurav Goel and Xichun Luo (Heriot Watt University) to simulate single point diamond turning (SPDT) of cubic SiC, as an application of ultra-precision machining. This work includes an MD model for quantification of wear of diamond tools involving graphitization.

The figures show the sp3 to sp2 transition and consequent wear of a diamond tool during SPDT of cubic SiC.

Molecular dynamics simulation model for the quantitative assessment of tool wear during single point diamond turning of cubic silicon carbide, S. Goel, X. Luo, R. L. Reuben, Comp Matl Sci, 51 402–408 (2011). (abstract)

Nanoparticle coating structure in the presence of solvent

This is work by Matthew Lane and Gary Grest (Sandia) to model the structure of nanoparticle coatings in solution. It demonstrates that small spherical nanoparticles -- coated with a simple polymer -- produce highly asymmetric coating arrangements even when one would expect otherwise. When particles are placed at the solvent surface, the asymmetric coatings are amplified and oriented by the surface.

The first 3 images show nanoparticles 2nm, 4nm, 8nm in diameter in various solvents: decane, implicit, water. The 4th image shows nanoparticles of various sizes at a water/vapor interface.

Spontaneous Asymmetry of Coated Spherical Nanoparticles in Solution and at Liquid-Vapor Interfaces, J. M. D. Lane and G. S. Grest, Phys Rev Lett, 104, 235501 (2010), http://dx.doi.org/10.1103/PhysRevLett.104.235501 (abstract)

Two-temperature model for electronic heat conduction

This is work by Carolyn Phillips (U Michigan) and Paul Crozier (Sandia) to add a two-temperature model to LAMMPS so that the effect of electronic heat conduction can be included in a classical MD atomic simulation. Including these effects strongly influences the rate and extent of annealing in LJ crystals. The new fix ttm for LAMMPS was tested on a single-component LJ crystal that easily recrystallizes and on a binary glass-forming LJ crystal that tends to retain permanent damage. Both systems were tested across a range of electron-ion coupling parameter values and electronic thermal conductivity values.

The first figure shows a hot damage spot in the center (red spheres) and a cold heat sink at the corners (blue spheres).

The second plot shows the effect of the electronic subsystem parameters on damage annealing.

An energy-conserving two-temperature model of radiation damage in single-component and binary Lennard-Jones crystals, C. L. Phillips and P. S. Crozier, J Chem Phys, 131, 074701:1-11 (2009). (abstract)

Atom-to-continuum coupling with the ATC package

This is work by Reese Jones (rjones at sandia.gov), Jeremy Templeton (jatempl at sandia.gov), and Jon Zimmerman (jzimmer at sandia.gov) at Sandia using their ATC package to couple finite element (FE) and molecular dynamics (MD) calculations. The package creates a FE mesh and passes information back and forth between the MD and FE representations of the problem each timestep.

The doc page for the fix atc command has further details and cites these 2 papers:

An atomistic-to-continuum coupling method for heat transfer in solids, G. J. Wagner, R. E. Jones, J. A. Templeton, and M. L. Parks, Special Issue of Computer Methods and Applied Mechanics, 197, 3351-3365 (2008). (abstract)

Calculation of stress in atomistic simulation, J. A. Zimmerman, E. B. Webb III, J. J. Hoyt, R. E. Jones, P. A. Klein, and D. J. Bammann, Special Issue of Modelling and Simulation in Materials Science and Engineering, 12, S319 (2004). (abstract)

Stress field around dislocations

This is work by Ed Webb (ebwebb at sandia.gov), Jon Zimmerman, and Steve Seel at Sandia to compare thermomechanical properties like stress computed in atomistic simulations to their continuum counterparts. Traditional elasticity theory would produce a singularity in stress at the core of a dislocation like that shown below, whereas atomic scale calculations and non-local elasticity theories avoid this shortcoming.

The figures show stress fields (sigma_xx) surrounding the core of an edge dislocation in EAM Al calculated using the discrete (top) and Hardy (bottom) expressions for sigma. The color contour maps represent peak tension in red at 5 GPa and peak compression in blue at -5 GPa. The Burgers vector direction (x) is horizontal in the figure.

Reconsideration of Continuum Thermomechanical Quantities in Atomic Scale Simulations, E. B. Webb III, J. A. Zimmerman, S. C. Seel, Mathematics and Mechanics of Solids, 13, 221-266 (2008). (abstract)

Water interacting with self-assembled monolayers

This is work by Matt Lane, Gary Grest, Mike Chandross, and Mark Stevens (all at Sandia) and Chris Lorenz (King's College, London). They studied the interaction of water with self-assembled monolayers (SAMs). Investigations included water penetration of damaged SAMs and water diffusion properties in nanoconfinement. The first snapshot shows the effects of water on SAM coatings with various sized regions of damage. The second shows only the water during penetration. The third snapshot shows water in nanoconfinement between two planar SAMs.

Water in Nanoconfinement between Hydrophilic Self-Assembled Monolayers, J. M. D. Lane, M. Chandross, M. J. Stevens, G. S. Grest, Langmuir, 24, 5209-5212 (2008). (abstract)

Water Penetration of Damaged Self-Assembled Monolayers, J. M. D. Lane, M. Chandross, C. D. Lorenz, M. J. Stevens, G. S. Grest, Langmuir, 24, 5734-5739 (2008). (abstract)

Coarse-grained self-assembly of lipids and PEG surfactants

This is work by Wataru Shinoda (AIST Tsukuba, Japan) in collaboration with Russell DeVane and Michael Klein (Temple U) to study self-assembly of organic molecules and their long timescale behavior using a novel coarse-grained parametrization scheme.

Both systems in these images have about 1 million particles. The image on the left is of a vesicle interacting with a lipid bilayer. The system on the right represents an aqueous surfactant solution run for 100 nanosecs before it undergoes a phase transition to the final ordered state.

Large-Scale Molecular Dynamics Simulations of Self-Assembling Systems, M. L. Klein and W. Shinoda, Science, 321, 798-800 (2008). (abstract)

Coarse-grained molecular modeling of non-ionic surfactant self-assembly, W. Shinoda, R. H. DeVane, M. L. Klein, Soft Matter, 4, 2453-2462 (2008). (abstract)

Spherical polyelectrolyte brushes

This is work by Ran Ni, Dapeng Cao and Wenchuan Wang in the Lab of Molecular and Materials Simulation at Beijing University of Chemical Technology and Arben Jusufi in Princeton University.

They studied the conformational behavior of a coarse-grained model of spherical polyelectrolyte brushes (SPB) in aqueous solutions containing oppositely charged linear polyelectrolytes (LPs). The snapshots show that with increasing concentration of LPs, the SPB undergoes swelling (left) -> collapse (middle) -> re-swelling (right).

Conformation of a Spherical Polyelectrolyte Brush in the Presence of Oppositely Charged Linear Polyelectrolytes, R. Ni, D. Cao, W. Wang, and A. Jusufi, Macromolecules, 41, 5477-5484 (2008). (abstract)

Coarse-grained block copolymer generation

This is work by Michel Perez, Olivier Lame, Fabien Leonforte, and Jean-Louis Barrat.

They use a versatile method, largely inspired by chemical "radical polymerization", to generate configurations of coarse-grained models for polymer melts. The two figures show snapshots of lamellar diblocks and triblocks. Equilibrium lamellar spacing depends on the incompatibility between the two (or three) polymers forming the block copolymer.

Polymer chain generation for coarse-grained models using radical-like polymerization, M. Perez, O. Lame, F. Leonforte and J.-L. Barrat, J Chem Phys, 128, 234904:1-11 (2008). (abstract)

Polyelectrolytes adsorption and brushes

This is work by Jan-Michael Carrillo (janmikel at gmail.com) and Andrey Dobrynin at the University of Connecticut.

The first picture shows snapshots of an adsorbed layer of hydrophobic polyelectrolytes on a hydrophilic substrate at different surface charge densities (increasing surface charge density from left to right).

The second plot is from the 2nd paper and is a diagram of states of spherical polyelectrolyte brushes : collapsed brushes (circles), bundle brushes (squares), star-like brushes (tilted squares), and micelle-like brushes (triangles). The dotted lines separating different conformational regimes are not actual phase transition lines, lB is the Bjerrum length of the system and Epsilon LJ is the strength of the monomer-monomer interaction.

Molecular Dynamics Simulations of Polyelectrolyte Adsorption, J.-M. Y. Carrillo and A. V. Dobrynin, Langmuir, 23, 2472-2482, (2007). (abstract)

Molecular Dynamics Simulations of Polyelectrolyte Brushes: From Single Chains to Bundles of Chains, D. J. Sandberg, J.-M. Y. Carrillo and A. V. Dobrynin, Langmuir, 23, 12716-12728 (2007). (abstract)

Stress in metal nanowires with twin boundaries

This is work by A-Jing Cao (chaoajing at lnm.imech.ac.cn) and Yue-Guang Wei (ywei at lnm.imech.ac.cn) at the Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences.

The picture on the left is the equilibrium structure of a nanowire constructed with a fivefold twinned grain boundary running down the axis of the wire. Tensile stress is applied. The picture in the middle shows the resulting dislocation pile-up. The picture on the right shows a different geometry where twin boundaries are oriented perpendicular to the axis of the nanowire. Atoms are colored according to the configuration of their neighbors; the visualization was done with the AtomEye program.

Formation of Fivefold Deformation Twins in Nanocrystalline Face-Centered-Cubic Copper Based on Molecular Dynamics Simulations, A. J. Cao and Y. G. Wei, Applied Physics Lett, 89, 041919 (2006). (abstract)

Atomistic simulations of the mechanical behavior of fivefold twinned nanowires, A. J. Cao and Y. G. Wei, Phys Rev B, 74, 214108 (2006). (abstract)

Deformation mechanisms of face-centered-cubic metal nanowires with twin boundaries, A. J. Cao, Y. G. Wei, and S. X. Mao, Applied Physics Letters, 90, 151909 (2007). (abstract)

Nanotip indentation of a coated surface

This is work by Mike Chandross (mechand at sandia.gov), Chris Lorenz, Mark Stevens, and Gary Grest at Sandia.

A 100A radius silica tip makes contact with a silica substrate, coated with a self-assembled monolayer of alkyl silanes for a study of friction and wear. The snapshots were made with VMD, and show deformation and damage to the coating layer due to the tip.

Nanotribology of Anti-Friction Coatings in MEMS, M. Chandross, C. D. Lorenz, G. S. Grest, M. J. Stevens, and E. B. Webb III, J Minerals, Metals, and Materials (JOM), 57, 55 (2005). (abstract)

Systematic study of the effect of disorder on nanotribology of self-assembled monolayers, M. Chandross, E. B. Webb III, M. J. Stevens, G. S. Grest, and S. H. Garofalini, Phys Rev Lett, 93, 166103/1-4 (2004). (abstract)

Simulations of Nanotribology with Realistic Probe Tip Models, M. Chandross, C. D. Lorenz, M. J. Stevens, G. S. Grest, Langmuir, 24, 1240 (2008). (abstract)

Surface Wetting by Polymer Nanodroplet

This is work by Dave Heine (heinedr at corning.com), Gary Grest (gsgrest at sandia.gov), and Ed Webb (ebwebb at sandia.gov) at Sandia.

Bead-spring polymer chains are placed on a surface in a droplet form. The degree of wetting that results depends on various parameters, including the surface interaction strength and chain length.

These images show cuts through the droplet for different simulation conditions. The blue surface allows for more wetting than the green.

Diverse Spreading Behavior of Binary Polymer Nanodroplets, D. R. Heine, G. S. Grest, and E. B. Webb III, Langmuir, 21, 7959 (2005). (abstract)

Liquid nanodroplets spreading on chemically patterned surfaces, G. S. Grest, D. R. Heine, and E. B. Webb III, Langmuir, 22, 4745-4749 (2006). (abstract)

Surface Wetting of Liquid Nanodroplets: Droplet Size Effects, D. R. Heine, G. S. Grest, and E. B. Webb III, Phys Rev Lett, 95, 107801 (2005). (abstract)

Shear of Cu bicrystal

This is work with Doug Spearot (gte432r at prism.gatech.edu) in David McDowell's group at Georgia Tech. A tilt bicrystal interface is formed by joining two Cu crystals and sheared via different deformation paths to study the defect formation and material response.

These images show the resulting strained system after deformation via 3 different paths. The top images color the atoms in each crystal in 2 shades of gray; the bottom images color atoms by the distance they moved from their initial positions.

Effect of Deformation Path Sequence on the Behavior of Nanoscale Copper Bicrystal Interfaces, D. E. Spearot, K. I. Jacob, D. L. McDowell, S. J. Plimpton, J Engr Materials and Technology, 127, 374-382 (2005). (abstract)

Solvated dendritic polymer structure

This is work by "Seung Soon Jang" (jsshys at wag.caltech.edu) in Bill Goddard's group at Caltech.

The 1st picture/paper are for a model they've developed of a dendrion diblock copolymer consisting of a dendritic polymer with a hydrophobic backbone. Such materials have interesting nanoscale structural and phase behavior.

The 2nd picture/paper are for simulations of amphiphilic bistable (2)rotaxane molecules which have controllable switching properties as their conformation changes.

The 3rd picture/paper are studies of the structure and surface concentrations of different surfactants in thin Newton black films.

The 1st picture shows the molecular structures of a diblock copolymer system at two different levels of water content. The 2nd picture illustrates conformational changes in a Langmuir monolayer of the rotaxane molecules. The 3rd picture shows film structure at varying surface concentrations (top) and film thicknesses (bottom).

Nanophase-segregation and water dynamics in the dendrion diblock copolymer formed from polyaryl ethereal dendrimer and linear PTFE, S. S. Jang, S.-T. Lin, T. Cagin, V. Molinero and W. A. Goddard III, J Phys Chem B, 109, 10154-10167 (2005). (abstract)

Molecular dynamics simulation of amphiphilic bistable (2)rotaxane Langmuir monolayer at air/water interface, S. S. Jang, Y. H. Jang, Y.-H. Kim, W. A. Goddard III, J. W. Choi, J. R. Heath, A. H. Flood, B. W. Laursen, and J. F. Stoddart, J Amer Chem Soc, 127, 14804 (2005). (abstract)

Structures and Properties of Newton Black Films Characterized Using Molecular Dynamics Simulations, S. S. Jang and W. A. Goddard III, J Phys Chem B, 110, 7992-8001 (2006). (abstract)

Metal solidification

This is work by Mark Asta's group at Northwestern and Jeff Hoyt (jjhoyt at sandia.gov) at Sandia. They've developed a simulation strategy for solidifying metals and metal alloys where the temperature of the system is carefully thermostatted so that the velocity of the interface can be accurately measured.

This snapshot is a liquid/solid interface in NiAl. See a movie of solidification on this page.

Calculation of alloy solid-liquid interfacial free energies from atomic-scale simulations, M. Asta, J. J. Hoyt, A. Karma, Phys Rev B, 66, 100101 (2002). (abstract)

Lipid membrane self-assembly and fusion

This is work by Mark Stevens (msteve at sandia.gov) at Sandia on the self-assembly of lipid bilayers and membrane fusion using an idealized bead-spring model for a 2-tail lipid molecule.

Head-head and head-solvent interactions are set to give hydrophilic behavior. Head-tail and tail-solvent interactions are hydrophobic. A 3d random ensemble of lipid molecules in a background solvent will spontaneously self-assemble into bilayers and vesicles as shown by these 2d slice views. When 2 vesicles are gently pushed together they can fuse as tails of individual lipid molecules straddle both membranes. The detailed fusion images were made with VMD.

Insights into the molecular mechanism of membrane fusion from simulation: Evidence for the association of splayed tails, M. J. Stevens, J. H. Hoh, T. B. Woolf, Phys Rev Lett, 91, 188102 (2003). (abstract)

Tensile pull on adhesive polymer chains

This is work by Scott Sides (swsides at mrl.ucsb.edu), Gary Grest (gsgrest at sandia.gov), and Mark Stevens (msteve at sandia.gov), all at Sandia, on adhesive properties of polymers.

The simulations are of melts of 500- and 1000-mer bead-spring chains. The systems range from 100-500K total monomers and are run for 10-20 million timesteps. In these snapshots of models with different parameters, the blue chains are the melt, red are tethered and unbroken chains, green are tethered and broken.

Large-scale simulation of adhesion dynamics for end-grafted polymers, S. W. Sides, G. S. Grest, M. J. Stevens, Macromolecules, 35, 566-573 (2002). (abstract)

Effect of end-tethered polymers on surface adhesion of glassy polymers, S. W. Sides, G. S. Grest, M. J. Stevens, S. J. Plimpton, Journal of Polymer Science, Part B (Polymer Physics), 42, 199-208 (2004). (abstract)

Crazing of entangled polymer chains

This is work by Joerg Rottler (now at Princeton) and Mark Robbins at JHU. The image shows a polymer glass that has been deformed into a craze at large strains. In the craze, polymers (~0.5 nm diameter) are bundled into an intricate load-bearing network of ~10 nm diameter fibrils. Crazing is largely responsible for the high fracture energy of glassy polymers.

Growth, microstructure, and failure of crazes in glassy polymers, J. Rottler and M. O. Robbins, Phys Rev E, 68, 011801 (2003). (abstract)

Jamming under tension in polymer crazes, J. Rottler and M. O. Robbins, Phys Rev Lett, 89, 195501 (2002). (abstract)

Cracks and crazes: On calculating the macroscopic fracture energy of glassy polymers from molecular simulations, J. Rottler, S. Barsky, M. O. Robbins, Phys Rev Lett, 89, 148304 (2002). (abstract)

Stress in metal nanowires

This is work by Min Zhou's group at Georgia Tech on modeling the effect of tensile stress at varying strain rates on single-crystal Cu nanowires of varying dimensions. In the image, atoms are colored to highlight defects and the transverse dimensions are drawn at an exaggerated scale.

Shear of large single-crystal metals

This is work with Mark Horstemeyer (mfhorst at me.msstate.edu) at Mississippi State (formerly at Sandia) and Mike Baskes (baskes at lanl.gov) at LANL to study stress/strain effects in large single-crystal metals samples. Simulations with up to 100M atoms were run. This image shows defect formation in a quasi-2d Ni sample undergoing fixed-end shear, where the z-dimension (into the image) is periodic but very thin. The black lines indicate atom displacements as the sample has sheared to the right.

This paper and related ones in the Metals section of this page have further details:

Computational nanoscale plasticity simulations using embedded atom potentials, M. F. Horstemeyer, M. I. Baskes, S. J. Plimpton, Theoretical and Applied Fracture Mechanics, 37, 49-98 (2001). (abstract)

Picture gallery from LAMMPS simulations