64 research outputs found
An efficient genetic algorithm for structure prediction at the nanoscale
We have developed and implemented a new global optimization technique based on a Lamarckian genetic algorithm with the focus on structure diversity. The key process in the efficient search on a given complex energy landscape proves to be the removal of duplicates that is achieved using a topological analysis of candidate structures. The careful geometrical prescreening of newly formed structures and the introduction of new mutation move classes improve the rate of success further. The power of the developed technique, implemented in the Knowledge Led Master Code, or KLMC, is demonstrated by its ability to locate and explore a challenging double funnel landscape of a Lennard-Jones 38 atom system (LJ38). We apply the redeveloped KLMC to investigate three chemically different systems: ionic semiconductor (ZnO)1â32, metallic Ni13 and covalently bonded C60. All four systems have been systematically explored on the energy landscape defined using interatomic potentials. The new developments allowed us to successfully locate the double funnels of LJ38, find new local and global minima for ZnO clusters, extensively explore the Ni13 and C60 (the buckminsterfullerene, or buckyball) potential energy surfaces
Structure prediction of (BaO)n nanoclusters for n⩜24 using an evolutionary algorithm
Knowing the structure of nanoclusters is relevant to gaining insight into their properties for materials design. Computational studies predicting their structure should aim to reproduce experimental results. Here, barium oxide was chosen for its suitability for both computational structure prediction and experimental structure determination. An evolutionary algorithm implemented within the KLMC structure prediction package was employed to find the thermodynamically most stable structures of barium oxide nanoclusters (BaO)n with n=4-18and24. Evolutionary algorithm runs were performed to locate local minima on the potential energy landscape defined using interatomic potentials, the structures of which were then refined using density functional theory. BaO clusters show greater preference than MgO for adopting cuts from its bulk phase, thus more closely resemble clusters of KF. (BaO)4, (BaO)6, (BaO)8, (BaO)10 and (BaO)16 should be magic number clusters and each are at least 0.03Â eV/BaO more stable than all other PBEsol local minima clusters found for the same size
Synthesis target structures for alkaline earth oxide clusters
Knowing the possible structures of individual clusters in nanostructured materials is an important first step in their design. With previous structure prediction data for BaO nanoclusters as a basis, data mining techniques were used to investigate candidate structures for magnesium oxide, calcium oxide and strontium oxide clusters. The lowest-energy structures and analysis of some of their structural properties are presented here. Clusters that are predicted to be ideal targets for synthesis, based on being both the only thermally accessible minimum for their size, and a size that is thermally accessible with respect to neighbouring sizes, include global minima for: sizes n = 9, 15, 16, 18 and 24 for (MgO)n; sizes n = 8, 9, 12, 16, 18 and 24 for (CaO) n ; the greatest number of sizes of (SrO) n clusters (n = 8, 9, 10, 12, 13, 15, 16, 18 and 24); and for (BaO) n sizes of n = 8, 10 and 16
Critical Temperature for -Particle Condensation within a Momentum Projected Mean Field Approach
Alpha-particle (quartet) condensation in homogeneous spin-isospin symmetric
nuclear matter is investigated. The usual Thouless criterion for the critical
temperature is extended to the quartet case. The in-medium four-body problem is
strongly simplified by the use of a momentum projected mean field ansatz for
the quartet. The self-consistent single particle wave functions are shown and
discussed for various values of the density at the critical temperature
Low energy neutrino scattering measurements at future Spallation Source facilities
In the future several Spallation Source facilities will be available
worldwide. Spallation Sources produce large amount of neutrinos from
decay-at-rest muons and thus can be well adapted to accommodate
state-of-the-art neutrino experiments. In this paper low energy neutrino
scattering experiments that can be performed at such facilities are reviewed.
Estimation of expected event rates are given for several nuclei, electrons and
protons at a detector located close to the source. A neutrino program at
Spallation Sources comprises neutrino-nucleus cross section measurements
relevant for neutrino and core-collapse supernova physics, electroweak tests
and lepton-flavor violation searches.Comment: 12 pages, 4 figures, 5 table
Are octahedral clusters missing on the carbon energy landscape?
We report a new class of carbon nanostructures at a lower sub-nano end of the size scale with a surprising stability, as compared to the well-known carbon fullerenes. The octahedral carbon clusters contain tetragonal rings, which, in spite of a common belief, prove to be an energy efficient means of plying graphene sheets to make three-dimensional spheroid shapes, similar to fullerenes. The two families of structures are shown to be competitive at small sizes (âŒ20 atoms) at room temperature, and for higher temperatures, at both small and large sizes (>200 atoms). Our calculations demonstrate that both vibrational and electronic spectra of these cluster families are similar, which thus might cloud their experimental identification. However, there is a sufficiently strong shift in vibrational frequencies below 160 and in the range of 600â800 cm^{-1}, which should help to identify different types of carbon clusters experimentally. We propose octahedral clusters and other structures containing tetragonal rings as viable structural elements and building units in inorganic chemistry and materials science of carbon along with fullerenes
Development of Interatomic Potentials for Supported Nanoparticles: The Cu/ZnO Case
We present a potential model that has been parametrized to reproduce accurately metalâmetal oxide interactions of Cu clusters supported on ZnO. Copper deposited on the nonpolar (101Ì
0) ZnO surface is investigated using the new pairwise CuâZnO interatomic potentials including repulsive BornâMayer CuâO and attractive Morse CuâZn potentials. Parameters of these interactions have been determined by fitting to periodic supercell DFT data using different surface terminations and Cu cluster sizes. Results of interatomic potential-based simulations show a good agreement both structurally and energetically with DFT data, and thus provide an efficient filter of configurations during a search for low DFT energy structures. Upon examining the low energy configurations of Cu clusters on ZnO nonpolar surfaces for a range of cluster sizes, we discovered why Cu islands are commonly observed on step edges on the (101Ì
0) surface but are rarely seen on terraces
What is the best or most relevant global minimum for nanoclusters? Predicting, comparing and recycling cluster structures with WASP@N
To address the question posed in the title, we have created, and now report details of, an open-access database of cluster structures with a web-assisted interface and toolkit as part of the WASP@N project. The database establishes a map of connectivities within each structure, the information about which is coded and kept as individual labels, called hashkeys, for the nanoclusters. These hashkeys are the basis for structure comparison within the database, and for establishing a map of connectivities between similar structures (topologies). The database is successfully used as a key element in a data-mining study of (MX)12 clusters of three binary compounds (LiI, SrO and GaAs) of which the database has no prior knowledge. The structures are assessed on the energy landscapes determined by the corresponding bulk interatomic potentials. Global optimisation, using a Lamarckian genetic algorithm, is used to search for low lying minima on the same energy landscape to confirm that the data-mined structures form a representative sample of the landscapes, with only very few structures missing from the close energy neighbourhood of the respective global minima
Thermodynamically accessible titanium clusters TiN, N = 2-32
We have performed a genetic algorithm search on the tight-binding interatomic potential energy surface (PES) for small TiN (N = 2-32) clusters. The low energy candidate clusters were further refined using density functional theory (DFT) calculations with the PBEsol exchange-correlation functional and evaluated with the PBEsol0 hybrid functional. The resulting clusters were analysed in terms of their structural features, growth mechanism and surface area. The results suggest a growth mechanism that is based on forming coordination centres by interpenetrating icosahedra, icositetrahedra and Frank-Kasper polyhedra. We identify centres of coordination, which act as centres of bulk nucleation in medium sized clusters and determine the morphological features of the cluster
Universality in Four-Boson Systems
We report recent advances on the study of universal weakly bound four-boson
states from the solutions of the Faddeev-Yakubovsky equations with zero-range
two-body interactions. In particular, we present the correlation between the
energies of successive tetramers between two neighbor Efimov trimers and
compare it to recent finite range potential model calculations. We provide
further results on the large momentum structure of the tetramer wave function,
where the four-body scale, introduced in the regularization procedure of the
bound state equations in momentum space, is clearly manifested. The results we
are presenting confirm a previous conjecture on a four-body scaling behavior,
which is independent of the three-body one. We show that the correlation
between the positions of two successive resonant four-boson recombination peaks
are consistent with recent data, as well as with recent calculations close to
the unitary limit. Systematic deviations suggest the relevance of range
corrections.Comment: Accepted for publication in special issue of Few-Body Systems devoted
to the Sixth Workshop on the Critical Stability of Quantum Few-Body Systems,
October 2011, Erice, Sicily, Ital
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