76,458 research outputs found
Workshop report. Linear-Scaling Ab Initio Calculations: Applications and Future Directions
No full textThe study of properties and of processes in materials, frequently hinges upon understanding phenomena which originate at the atomic level. In such cases the accurate description of the interactions between large numbers of atoms is critical and in turn requires the accurate description of the electrons which play a crucial role in the bonding of atoms into molecules, surfaces and solids. This can only be achieved by solving the equations of quantum mechanics. These
equations are too complicated to solve exactly; however their solutions can be approximated by computational techniques. The most accurate ? but also most computationally demanding ? are the “ab initio” techniques which do not use any empirical adjustable parameters. Amongst them, the Density Functional Theory (DFT) formulation of quantum mechanics stands out as an excellent compromise between accuracy and computational efficiency. However, the applicability of ab initio techniques is severely limited by poor scaling: the computational effort needed to perform an ab initio calculation increases with (at least) the third power of the number of atoms, N. This cubic-scaling bottleneck limits the number of atoms we can study to a few hundred at most, even on parallel supercomputers. To overcome this length-scale limitation, a number of researchers worldwide have been pioneering the development of a novel class of ab initio methods with linear-scaling or “Order N” (O(N)) computational cost which nevertheless retain the same high level of accuracy as the conventional approaches. While physically motivated, such methods have proved particularly hard to develop as they introduce highly non-trivial localisation constraints. Nevertheless, many major obstacles have been overcome and a number of O(N)
methods (SIESTA, CONQUEST, ONETEP, etc.) for ground state DFT calculations on systems with a gap (e.g. molecules, semiconductors and insulators) are now available and have reached a state of maturity that allows them to be used to study ”real” materials. The particular focus of this workshop is therefore to look forward to what can be achieved in the next few years. Our aim is twofold: (1) As O(N) methods are currently extending the applicability of DFT calculations
to problems involving biomolecules and nanostructures they are leading to completely new levels of understanding of these systems. This CECAM meeting will give us the opportunity to make an appraisal of such large-scale simulations and their potential to connect more directly to experiments. (2) We also want to examine the options for extending linear-scaling to problems that cannot be treated by ground-state DFT but require other, more complex approaches. These include methods for treating metallic systems, excited states and wavefunction-based theories for including electronic correlation. Finding ways to transform these methods to linear-scaling
cost, and hence extent their applicability to the nano-scale, is the next big challenge that the community of developers of large-scale electronic structure methods is beginning to face. We
hope that this workshop will stimulate these major new O(N) methodological developments by bringing together the leading groups in the development of O(N) DFT methods with the leading
groups in the development of metal and excited-state or wavefunction-based methods. Strong emphasis during the workshop will be given to discussion in order to promote the exchange
of ideas between different communities (Physics, Chemistry, Materials Science, Biochemistry) which are all interested in large-scale applications with ab initio accuracy but are approaching
them from different perspectives
The vanishing author in computer-generated works: a critical analysis of recent Australian case law
No full textAbstract
The use of software is ubiquitous in the creation of many copyright works, yet the requirement in copyright law that every work have a human author who engages in independent intellectual effort means that its use may prevent copyright subsistence. Several recent Australian cases have refocused attention on authorship as an essential criterion of copyright subsistence, and these cases suggest that much computer-produced output may be authorless and thus lack copyright protection. This article, the first in a two-part series, analyses how each case deals with the question of authorship of computer-produced works and why the use of software diminishes copyright protection for a significant number of computer-generated works. The article critiques the application of conventional notions of human authorship developed in the pre-computer age to modern productions and suggests alternative approaches to authorship that satisfy both the major objectives of copyright policy and the need to adapt to the computer age. The article argues that, without a broader judicial approach to authorship of computer-generated works, Parliament must remedy the lacuna in protection for these ‘authorless’ works. Possible solutions for reform are suggested. In a forthcoming article, the author comprehensively examines those reform proposals
Quantification and spatial comparisons of pRNFL and segmented macular layers in AQP4-Ab-positive ON and AQP4-Ab-negative ON eyes without an ON attack for 6 months.
No full textFig 3. Analysis of pRNFL thickness and segmented macular layer volume. Fig 3A, 3B and 3C show the statistical results of average pRNFL thickness, GCIP volume and INL volume, when controlling BCVA, inter-eye correlation, episodes of ON attack and disease duration. Fig 3D shows that pRNFL thickness in N, NS, NI and TI quadrants in AQP4-Ab-positive ON eyes decreased when compared with AQP4-Ab-negitive ON eyes. In contrast to AQP4-Ab-negative ON eyes, Fig 3E shows that GCIP thickness in AQP4-Ab-positive ON eyes decreased in the inferior sector of the inner circle. Fig 3F shows that there was no significant difference in the INL thickness for each sector between the AQP4-Ab-positive ON cohort and the AQP4-Ab-negative ON cohort. AQP4-Ab+/ON: AQP4-Ab-positive ON; AQP4-Ab-/ON: AQP4-Ab- negative ON; *PP<0.01.</p
Long-Range Complex in the HC3 N + CN Potential Energy Surface: Ab Initio Calculations and Intermolecular Potential
No full textIn this work we characterize an initial van der Waals adduct in the potential energy surface of reaction between cyanoacetylene HC3 N and the cyano radical. The geometry of the CN-HC3 N adduct has been optimized through calculations employing ab initio methods. Results show that the energy of the adduct lays below the reactants. Additionally, a saddle point that connects the adduct to an important intermediate of the PES has been localized, with energy below the reactants. Calculations of the intermolecular potential have been performed and results show that the energy of the van der Waals adduct is higher than estimated with the ab initio methods
Fabrication and ab initio study of downscaled graphene nanoelectronic devices
No full textIn this paper we first present a new fabrication process of downscaled graphene nanodevices based on direct milling of graphene using an atomic-size helium ion beam. We address the issue of contamination caused by the electron-beam lithography process to pattern the contact metals prior to the ultrafine milling process in the helium ion microscope (HIM). We then present our recent experimental study of the effects of the helium ion exposure on the carrier transport properties. By varying the time of helium ion bombardment onto a bilayer graphene nanoribbon transistor, the change in the transfer characteristics is investigated along with underlying carrier scattering mechanisms. Finally we study the effects of various single defects introduced into extremely-scaled armchair graphene nanoribbons on the carrier transport properties using ab initio simulation
Electron-N₂⁺ scattering and dynamics
No full textMolecular nitrogen, N₂, is the most abundant molecule in the terrestrial atmosphere. Its cation N₂⁺ is therefore prevalent in the earth's ionosphere as well as in nitrogen plasmas produced for reasons varying from lightning strikes to combustion. Any model which seeks to describe plasmas in air must contain a description of nitrogen ion chemistry. Despite this, there is a distinct paucity of data describing electron-N₂⁺ interactions and the resultant bound and quasi-bound electronic structure of N₂. The characterisation of these states is essential for describing dissociative recombination which is the main destroyer of molecular ions in a plasma. This thesis aims to alleviate this problem by performing extensive ab initio R-matrix calculations to create a comprehensive map of the highly-excited electronic structure of N₂ which can the be used to perform a dissociative recombination cross-section calculation. Potential energy curves were found by performing resonant and bound state calculations for all singlet and triplet molecular symmetries of N₂ up to l ≤ 4. The use of a dense grid meant that highly-excited electronic states could be found with an unprecedented level of detail. Many of the states were previously unknown. A new fitting method was developed for the characterisation of resonant states using the time-delay method. It was shown that whilst the R-matrix method is not competitive with conventional quantum chemistry techniques for low lying valence states, it is particularly appropriate for highly-excited states, such as Rydberg states. The data gained from these calculations was then used as an input for a multichannel quantum defect theory calculation of a dissociative recombination cross-section. A description is given of how to prepare the data from the R-matrix calculation for input into a multichannel quantum defect theory dissociative recombination cross-section calculation. Cross-sections were found for v=0-3 including three ionic cores. Whilst previous studies of dissociative recombination using R-matrix data required some empirical intervention, the cross-section found in this thesis is completely ab initio and is in good agreement with experiment
New ab initio potential energy surface and quantum dynamics of the reaction H(2S) + NH(X3Σ-) → N(4S) + H2
No full textNew ab initio potential energy surface and quantum dynamics of the reaction H(2S) + NH(X3Σ-) → N(4S) + H
Ab initio simulations of iron-nickel alloys at Earth's core conditions
No full textWe report ab initio density functional theory calculations on iron–nickel (FeNi) alloys at conditions representative of the Earth's inner core. We test different concentrations of Ni, up to ∼39 wt% using ab initio lattice dynamics, and investigate the thermodynamic and vibrational stability of the three candidate crystal structures (bcc, hcp and fcc). First of all, at inner core pressures, we find that pure Fe transforms from the hcp to the fcc phase at around 6000 K. Secondly, in agreement with low pressure experiments on Fe–Ni alloys, we find the fcc structure is stabilised by the incorporation of Ni under core pressures and temperatures. Our results show that the fcc structure may, therefore, be stable under core conditions depending on the temperature in the inner core and the Ni content. Lastly, we find that within the quasi-harmonic approximation, there is no stability field for FeNi alloys in the bcc structure under core conditions
Ab-initio computation of superconducting properties of elemental superconductors and MgB2
No full textWe present ab-initio predictions of superconducting properties of some elemental superconductors and of MgB2, based on the Super-Conducting Density Functional theory (SC-DFT). This formalism allows a description of superconducting properties at thermal equilibrium by means of three “densities”: the ordinary electron density, the superconducting order parameter, and the diagonal of the nuclear N-body density matrix. These quantities are determined through self-consistent solutions of Bogoliubov-de Gennes Kohn-Sham like equations, involving exchange-correlation potentials which are universal functionals of the three above-mentioned quantities. By means of approximate expressions for the relevant functionals, we obtain an ab-initio description of the superconducting state, completely free of empirical parameters. The results of our present implementation of SC-DFT for selected materials are discussed in terms of superconducting energy gap, critical temperature and specific heat, and compared with experiments.</p
Ab-initio computation of superconducting properties of elemental superconductors and MgB2
No full textWe present ab-initio predictions of superconducting properties of some elemental superconductors and of MgB2, based on the Super-Conducting Density Functional theory (SC-DFT). This formalism allows a description of superconducting properties at thermal equilibrium by means of three “densities”: the ordinary electron density, the superconducting order parameter, and the diagonal of the nuclear N-body density matrix. These quantities are determined through self-consistent solutions of Bogoliubov-de Gennes Kohn-Sham like equations, involving exchange-correlation potentials which are universal functionals of the three above-mentioned quantities. By means of approximate expressions for the relevant functionals, we obtain an ab-initio description of the superconducting state, completely free of empirical parameters. The results of our present implementation of SC-DFT for selected materials are discussed in terms of superconducting energy gap, critical temperature and specific heat, and compared with experiments.</p
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