Showing papers on "Cluster (physics) published in 2004"
TL;DR: It is demonstrated that in both model systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters, which is relevant for nucleation processes during protein crystallization, protein or DNA self-assembly.
Abstract: Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.
967 citations
TL;DR: Experimental evidence is presented for a very stable cluster corresponding to Al13I–, which features a structurally unperturbed Al13– core and a region of high charge density on the aluminum vertex opposite from the iodine atom.
Abstract: Al13- is a cluster known for the pronounced stability that arises from coincident closures of its geometric and electronic shells. We present experimental evidence for a very stable cluster corresponding to Al13I-. Ab initio calculations show that the cluster features a structurally unperturbed Al13- core and a region of high charge density on the aluminum vertex opposite from the iodine atom. This ionically bound magic cluster can be understood by considering that Al13 has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides provide a sound explanation for our chemical observations.
484 citations
TL;DR: In this article, the basic principles of the physics of quantum halo systems, defined as bound states of clusters of particles with a radius extending well into classically forbidden regions, are discussed.
Abstract: This article provides an overview of the basic principles of the physics of quantum halo systems, defined as bound states of clusters of particles with a radius extending well into classically forbidden regions. Exploiting the consequences of this definition, the authors derive the conditions for occurrence in terms of the number of clusters, binding energy, angular momentum, cluster charges, and excitation energy. All these quantities must be small. The article discusses the transitions between different cluster divisions and the importance of thresholds for cluster or particle decay, with particular attention to the Efimov effect and the related exotic states. The pertinent properties can be described by the use of dimensionless variables. Then universal and specific properties can be distinguished, as shown in a series of examples selected from nuclear, atomic, and molecular systems. The neutron dripline is especially interesting for nuclei and negative ions for atoms. For molecules, in which the cluster division comes naturally, a wider range of possibilities exists. Halos in two dimensions have very different properties, and their states are easily spatially extended, whereas Borromean systems are unlikely and spatially confined. The Efimov effect and the Thomas collapse occur only for dimensions between 2.3 and 3.8 and thus not for 2. High-energy reactions directly probe the halo structure. The authors discuss the reaction mechanisms for high-energy nuclear few-body halo breakup on light, intermediate, and heavy nuclear targets. For light targets, the strong interaction dominates, while for heavy targets, the Coulomb interaction dominates. For intermediate targets these processes are of comparable magnitude. As in atomic and molecular physics, a geometric impact-parameter picture is very appropriate. Finally, the authors briefly consider the complementary processes involving electroweak probes available through beta decay, electromagnetic transitions, and capture reactions.
481 citations
TL;DR: The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis as discussed by the authors, including X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations.
Abstract: The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the μG level, with values up to tens of μG at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant throughout the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster ...
441 citations
TL;DR: A model in which particles interact with short-ranged attractive and long-ranged repulsive interactions is studied, in an attempt to model the equilibrium cluster phase recently discovered in sterically stabilized colloidal systems in the presence of depletion interactions.
Abstract: We study a model in which particles interact with short-ranged attractive and long-ranged repulsive interactions, in an attempt to model the equilibrium cluster phase recently discovered in sterically stabilized colloidal systems in the presence of depletion interactions. At low packing fractions, particles form stable equilibrium clusters which act as building blocks of a cluster fluid. We study the possibility that cluster fluids generate a low-density disordered arrested phase, a gel, via a glass transition driven by the repulsive interaction. In this model the gel formation is formally described with the same physics of the glass formation.
435 citations
TL;DR: In this article, the authors review the main issues that have led to the knowledge on magnetic fields in clusters of galaxies, and compare the observational results with the prediction from numerical simulations.
Abstract: The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the $\mu$G level, with values up to tens of $\mu$G at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant through the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster radius with a rough dependence on the thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate over a wide range of spatial scales with values from a few kpc up to hundreds kpc. Important information on the cluster field are obtained by comparing the observational results with the prediction from numerical simulations. The origin of cluster magnetic fields is still debated. They might originate in the early Universe, either before or after the recombination, or they could have been deposited in the intracluster medium by normal galaxies, starburst galaxies, or AGN. In either case, magnetic fields undergo significant amplification during the cluster merger processes.
431 citations
TL;DR: Among the cluster imaging methods, AH clustering (Ward's algorithm) proved to be the best method in terms of tissue structure differentiation.
393 citations
TL;DR: In this article, the authors present VLA HI observations at 20" = 1.5 kpc resolution of the highly inclined, HI-deficient Virgo cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing ICM stripping.
Abstract: We present VLA HI observations at ~20"=1.5 kpc resolution of the highly inclined, HI-deficient Virgo cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing ICM-ISM stripping. HI is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the HI distribution in the disk is sharply truncated and the only HI is extraplanar, and all on the northwest side. The kinematics and the morphology of the HI appear more consistent with ongoing stripping, and less consistent with gas fall-back which may occur long after peak pressure. Much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The SW side of the galaxy has less HI in the disk but more HI in the halo, suggesting more effective gas removal on the side of the galaxy which is rotating into the ICM wind. The galaxy is 3.3 degrees ~800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure may be significantly stronger than standard values, due to large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The HI and H-alpha distributions are similar, implying that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the HI.
380 citations
TL;DR: A new family of magic cluster structures is found by genetic global optimization, whose results are confirmed by density functional calculations, which can have high melting points, large energy gaps, and (in the case of Ag-Ni) nonzero magnetic moments.
Abstract: A new family of magic cluster structures is found by genetic global optimization, whose results are confirmed by density functional calculations. These clusters are Ag-Ni and Ag-Cu nanoparticles with an inner Ni or Cu core and an Ag external shell, as experimentally observed for Ag-Ni, and present a polyicosahedral character. The interplay of the core-shell chemical ordering with the polyicosahedral structural arrangement gives high-symmetry clusters of remarkable structural, thermodynamic, and electronic stability, which can have high melting points (they melt higher than pure clusters of the same size), large energy gaps, and (in the case of Ag-Ni) nonzero magnetic moments.
356 citations
TL;DR: The clustering problem is the task of making the fewest changes to the edge set of an input graph so that it becomes a cluster graph, and it is shown that Cluster Editing is NP-complete, Cluster Deletion isNP-hard to approximate to within some constant factor, and Cluster Completion is polynomial.
308 citations
TL;DR: This minireview highlights some of the most recent advances on a particular type of hybrids where the organic and inorganic components are connected by an imido linkage.
Abstract: Hybrid materials based on covalently linked inorganic polyoxometalates (POMs) and organic species containing a delocalized pi system have drawn increasing attention. These hybrids, traditionally prepared by cluster assembly approaches that lack predictability and controllability, can now be synthesized through common organic reactions by using organically functionalized POM clusters as building blocks. This minireview highlights some of the most recent advances on a particular type of hybrids where the organic and inorganic components are connected by an imido linkage.
TL;DR: In this article, the authors view clusters as a specific spatial configuration of the economy suitable for the creation, transfer and usage of knowledge, and investigate how the modern exchange-economy becomes orga...
Abstract: This paper views clusters as a specific spatial configuration of the economy suitable for the creation, transfer and usage of knowledge. It investigates how the modern exchange-economy becomes orga...
TL;DR: In this article, the authors examined the possibility of measuring the dark energy equation-of-state parameter w within the context of any uncertain redshift evolution of galaxy cluster structure and showed that including the redshift-averaged cluster power spectrum (cl) and direct mass measurements of 100 clusters helps tremendously in reducing cosmological parameter uncertainties.
Abstract: We examine the prospects for measuring the dark energy equation-of-state parameter w within the context of any uncertain redshift evolution of galaxy cluster structure (building on our earlier work) and show that including the redshift-averaged cluster power spectrum (cl) and direct mass measurements of 100 clusters helps tremendously in reducing cosmological parameter uncertainties. Specifically, we show that when combining the redshift distribution and the power spectrum information for a particular X-ray survey (DUET) and two Sunyaev-Zel'dovich effect surveys (South Pole Telescope and Planck), the constraints on the dark energy equation-of-state w can be improved by roughly a factor of 4. Because surveys designed to study the redshift distribution of clusters will have all the information necessary to construct cl, the benefit of adding cl in reducing uncertainties comes at no additional observational cost. Combining detailed mass studies of 100 clusters with the redshift distribution improves the parameter uncertainties by a factor of 3-5. The data required for these detailed mass measurements—assumed to have 1 σ uncertainties of 30%—are accumulating in the XMM-Newton and Chandra archives. The best constraints are obtained when one combines both the power spectrum constraints and the mass measurements with the cluster redshift distribution; when using the survey to extract the parameters and evolution of the mass-observable relations, we estimate uncertainties on w of ~4%-6%. These parameter constraints are possible with cluster surveys that are carried out over large, contiguous regions of the sky, so that the cluster power spectrum can be reasonably measured. In combination with cosmic microwave background or distance measurements that have different parameter degeneracies, cluster studies of dark energy will provide enhanced constraints and allow for cross-checks of systematics.
TL;DR: In this paper, a density functional theory with a plane wave basis set was used to study the stability of planar and layered platinum clusters, and the results showed that planar platinum clusters of up to nine atoms are as stable as their three-dimensional isomers.
Abstract: Platinum clusters of up to 55 atoms were studied using density functional theory with a plane wave basis set. The results show that planar platinum clusters of up to nine atoms are as stable as their three-dimensional isomers and the six-atom planar cluster is, surprisingly, more stable than its three-dimensional isomers. Among the three-dimensional platinum clusters investigated in this work, the layered clusters are found to be as stable as their spherical close-packed isomers. The high stability of planar and layered clusters suggests that it is easy to grow a platinum monolayer or multilayer. The existence of many energetically possible isomers shows a fluxional structural characteristic of platinum clusters. The effect of the spin−orbit coupling was investigated, and the results show that the relative stability of the Pt clusters is not affected although the binding energy of the cluster increases if the spin−orbit coupling is included in the calculation. Most of the platinum clusters studied here sh...
INAF1
TL;DR: In this paper, the authors study the iron content of 22 nearby hot clusters observed with BeppoSAX and find that the global iron abundance is tightly related to the cluster luminosity and that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron vs. luminosity, and luminosity vs. temperature correlations.
Abstract: In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX .W e fi nd that the global iron mass of clusters is tightly related to the cluster luminosity and that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron mass vs. luminosity and luminosity vs. temperature correlations. The iron mass is found to scale linearly with the intracluster gas mass, implying that the global iron abundance in clusters is roughly constant. This result suggests that enrichment mechanisms operate at a similar rate in all clusters. By employing population synthesis and chemical enrichment models, we show that the iron mass associated with the abundance excess which is always found in the centre of cool core clusters can be entirely produced by the brightest cluster galaxy (BCG), which is always found at the centre of cool core clusters. The iron mass associated with the excess, the optical magnitude of the BCG and the temperature of the cluster are found to correlate with one another suggesting a link between the properties of the BCG and the hosting cluster. These observational facts lends strength to current formation theories which envisage a strong connection between the formation of the giant BCG and its hosting cluster.
22 Jan 2004
TL;DR: In this paper, a framework to analyze cluster performance is developed, drawing from different schools that deal with clusters, and central in the framework is a distinction to variables of cluster performance related to the structure of a cluster.
Abstract: This PhD thesis deals with the performance of clusters. Even though cluster studies are numerous, a coherent framework to analyze cluster performance is lacking. In this thesis, such a framework is developed, drawing from different schools that deal with clusters. Central in the framework is a distinction to variables of cluster performance related to the structure of a cluster and variables related to the governance of a cluster. Four structure variables - agglomeration ands disagglomeration forces, internal competition, heterogeneity of the cluster and the level of entry and exit barriers - and four governance related variables - the presence of trust, the presence of intermediaries, the presence of leader firms and the quality of collective action regimes - are identified and discussed. The validity of these variables is confirmed in the three case studies, of the port clusters of Rotterdam, Durban, and the lower Mississippi. The strengths and weaknesses of the three port clusters, the importance of the variables discussed above and opportunities for policy and management to improve the performance of clusters are discussed.The results of this study are relevant for cluster scholars and for scholars specializing in port studies. The thesis is also relevant for (port) cluster managers and for managers of firms in (port) clusters, since implications of this study for policy and management in (port) clusters are discussed.
TL;DR: In this paper, the authors present an analysis of K-band stellar distributions for the young stellar clusters GGD 12-15, IRAS 20050+2720, and NGC 7129.
Abstract: We present an analysis of K-band stellar distributions for the young stellar clusters GGD 12-15, IRAS 20050+2720, and NGC 7129. We find that the two deeply embedded clusters, GGD 12-15 and IRAS 20050+2720, are not azimuthally symmetric and show a high degree of structure which traces filamentary structure observed in 850 micron emission maps. In contrast, the NGC 7129 cluster is circularly symmetric, less dense, and anti-correlated to 850 micron emission, suggesting recent gas expulsion and dynamical expansion have occured. We estimate stellar volume densities from nearest neighbor distances, and discuss the impact of these densities on the evolution of circumstellar disks and protostellar envelopes in these regions.
TL;DR: In this paper, the isolated tetragonal rule was optimized by molecular orbital calculations to obtain a bandgap energy of 5.1 eV, which is a little smaller than that of B36N36 cluster.
TL;DR: In this paper, the authors suggest that areas where domestic burglary clusters shift over time, such that the location of clusters is not predictable over periods of three or more months, and the implications of the findings for hot-spotting and crime prevention are discussed.
Abstract: This research, inspired by the precepts of optimal foraging theory, suggests that areas where domestic
burglaries cluster shift over time, such that the location of clusters is not predictable over periods of
three or more months. However, although clusters do not remain in the same location over time (i.e.
are not stable over prolonged periods) they do tend to move in a ‘slippery’ manner, moving to
nearby areas at successive points in time. The implications of the findings for hot-spotting and
crime prevention are discussed.
TL;DR: In this paper, the authors used broad-band images from the Hubble Space Telescope (HST) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51 and obtained age, mass, extinction, and effective radius estimates for 1152 star clusters.
Abstract: We use archival {\it Hubble Space Telescope} observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of $\sim 7.3 \times 8.1$ kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). Using this dataset we find: {\it i}) that the cluster formation rate seems to have had a large increase $\sim$ 50-70 Myr ago, which is coincident with the suggested {\it second passage} of its companion, NGC 5195, {\it ii}) a large number of extremely young ($<$ 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next $\sim$10 Myr, and {\it iii)} that the distribution of cluster sizescan be well approximated by a power-law with exponent, $ -\eta = -2.2 \pm 0.2$, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters.
TL;DR: In this paper, a joint computational and experimental effort was directed towards the understanding of large solid water clusters, which included structure optimizations and calculations of OH stretch spectra for select sizes in the range n = 20 − 931.
Abstract: A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n = 20–931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasi-spherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mos...
TL;DR: In this paper, a statistical magneto-turbulent model was developed to investigate the connection between thermal and non-thermal properties of the ICM and the nonthermal emission from clusters.
Abstract: The most important evidences for non-thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scale observed in a growing number of massive clusters. A promising possibility to explain giant radio halos is given by the presence of relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events. With the aim to investigate the connection between thermal and non-thermal properties of the ICM, in this paper we develope a statistical magneto-turbulent model which describes in a self-consistent way the evolution of the thermal ICM and that of the non-thermal emission from clusters. Making use of the extended Press & Schechter formalism, we follow cluster mergers and estimate the injection rate of the fluid turbulence generated during these energetic events. We then calculate the evolution of the spectrum of the relativistic electrons in the ICM during the cluster life by taking into account both the electron-acceleration due to the merger-driven turbulence and the relevant energy losses of the electrons. We end up with a synthetic population of galaxy clusters for which the evolution of the ICM and of the non-thermal spectrum emitted by the accelerated electrons is calculated. The generation of detectable non-thermal radio and hard X-ray emission in the simulated clusters is found to be possible during major merger events for reliable values of the model parameters. In addition the occurrence of radio halos as a function of the mass of the parent clusters is calculated and compared with observations. In this case it is found that the model expectations are in good agreement with observations.
TL;DR: In this article, the authors applied the discrete path sampling approach to analyze the dynamics of several atomic and molecular clusters and calculated permutational isomerization rates for icosahedral atomic clusters containing 13 and 55 atoms.
Abstract: The discrete path sampling approach is applied to analyse the dynamics of several atomic and molecular clusters. Permutational isomerization rates are first calculated for icosahedral atomic clusters containing 13 and 55 atoms. The transformation between decahedral and icosahedral morphologies of a 75-atom cluster is then investigated, for which the potential energy surface has double funnel character. The final system considered is a cluster of twenty water molecules treated using a rigid molecule pair potential. Detailed analysis of the database of stationary points produced by the initial sampling is used to investigate the accuracy of the two-state description in each case. A clear deviation from two-state behaviour occurs for (H2O)20, where low-lying intervening minima exist.
TL;DR: In this article, the authors simulate the dynamics of fractal star clusters, in order to investigate the evolution of substructure in recently formed clusters, and find that the velocity dispersion is the key parameter determining the survival of substructures.
Abstract: We simulate the dynamics of fractal star clusters, in order to investigate the evolution of substructure in recently formed clusters. The velocity dispersion is found to be the key parameter determining the survival of substructure. In clusters with a low initial velocity dispersion, the ensuing collapse of the cluster tends to erase substructure, although some substructure may persist beyond the collapse phase. In clusters with virial ratios of 0.5 or higher, initial density substructure survives for several crossing times, in virtually all cases. Even an initially homogeneous cluster can develop substructure, if it is born with coherent velocity dispersion.
These results suggest that the simple initial conditions used for many sophisticated N-body simulations could be missing a very important and dramatic phase of star cluster evolution.
TL;DR: In this paper, the conditions which should be fulfilled to grow chains of nanosized noble metal clusters on DNA templates according to a selectively heterogeneous, template-controlled mechanism were investigated, and the effect of binding Pt(II) complexes to the DNA was investigated by UV−vis spectroscopy, electrophoresis experiments, and scanning force microscopy, revealing that the base stacking along the DNA molecule is significantly distorted but the double-stranded DNA configuration is retained.
Abstract: We investigate the conditions which should be fulfilled to grow chains of nanosized noble metal clusters on DNA templates according to a selectively heterogeneous, template-controlled mechanism. A long incubation of double-stranded DNA molecules with Pt(II) complexes is necessary to obtain a template-directed formation of thin and uniform cluster chains after chemical reduction of the DNA/salt solution. Without this “activation” step, DNA acts as a nonspecific capping agent for the formed clusters and does not hinder the formation of random cluster aggregates. The effect of binding Pt(II) complexes to the DNA is investigated by UV−vis spectroscopy, electrophoresis experiments, and scanning force microscopy, revealing that the base stacking along the DNA molecule is significantly distorted but the double-stranded DNA configuration is retained. Citrate ions can be used as additional stabilizers for the heterogeneously grown metal clusters, leading to significantly more regular metal cluster chains. After a ...
TL;DR: In this paper, the effect of catalyst particle size on the growth mechanism and structure of single-walled carbon nanotubes (SWNTs) was studied using an empirical potential energy surface.
Abstract: The molecular dynamics method, based on an empirical potential energy surface, was used to study the effect of catalyst particle size on the growth mechanism and structure of single-walled carbon nanotubes (SWNTs). The temperature for nanotube nucleation (800-1100 K), which occurs on the surface of the cluster, is similar to that used in catalyst chemical vapor deposition experiments, and the growth mechanism, which is described within the vapor-liquid-solid model, is the same for all cluster sizes studied here (iron clusters containing between 10 and 200 atoms were simulated). Large catalyst particles, which contain at least 20 iron atoms, nucleate SWNTs that have a far better tubular structure than SWNTs nucleated from smaller clusters. In addition, the SWNTs that grow from the larger clusters have diameters that are similar to the cluster diameter, whereas the smaller clusters, which have diameters less than 0.5 nm, nucleate nanotubes that are approximately 0.6-0.7 nm in diameter. This is in agreement with the experimental observations that SWNT diameters are similar to the catalyst particle diameter, and that the narrowest free-standing SWNT is 0.6-0.7 nm.