[en] Multiply charged cluster ions are produced via adiabatic expansion and subsequent electron impact ionization. In mass spectra of van der Waals or hydrogen-bonded systems multiply charged cluster ions are only observed when their size n (number of molecules in the cluster) exceeds a critical size nsup(zsup(*)) depending on the charge state z. In both systems investigated, ammonia and carbon dioxide, doubly charged cluster ions above the critical size (Co₂: nsup(2sup(*)) = 44/1/; NH₃: nsup(2sup(*)) - 51/2/) dissociate in the experimentally accessible time window. They mainly lose one monomer and, roughly ten times less probably, two monomers. An analysis of the dissociation channels for triply charged NHsub3 clusters (size approx=110-120) shows an extremly asymmetric dissociation ('fission') resulting in doubly charged cluster ions with about 90% of the initial mass. (Author)

[en] Electron interaction with van der Waals clusters produced by supersonic expansion has been studied with a double focussing mass spectrometer. In particular we have investigated here, (i) the production and stability of singly charged Ar cluster ions (evolution of magic numbers in the mass spectrum; dissociation channels for unimolecular (metastable) decay as α function of cluster size (up to n = 25) and stagnation conditions), and (ii) the electron attachment to O₂ and CO₂ clusters discovering a highly efficient attachment channel with electrons close to zero energy. (Author)

[en] A careful search has been made for dinegatively charged cluster ions. (O₂)₃^2- and larger homologs have been observed for the first time.They were produced via electron attachment with near zero energy electrons to neutral O₂ clusters formed by nozzle expansion. (Author)

[en] Electron attachment to CO₂ clusters formed by nozzle expansion was investigated in a crossed molecular-beam--electron-impact--mass spectrometer system. In addition to cluster ions previously observed at 3--4 eV electron energy we observe presently cluster ions produced at around zero electron energy. Some of these ions are likely produced by a less dissociative production mechanism allowing the probing of cluster beams with better reliability than previously

[en] Size distributions of fragments from multiply charged clusters have been determined. On a time scale of 0.1 ms with respect to ionization, triply charged CO₂ clusters of size nroughly-equal114 are found to fission extremely asymmetrically, their doubly charged fragments carrying 92% of the mass. For doubly charged CO₂ clusters, however, evaporation of neutral monomers is the only channel for delayed dissociation. A model, based on the liquid-drop approximation, can quantitatively account for the lower size limits of multiply charged clusters as well as for the distribution of fission fragments from (CO₂)/sub roughly-equal114/ ³⁺. .AE

[en] A double focussing mass spectrometer in combination with an improved electron impact ionization source was used to determine absolute partial and total ionization cross section functions from threshold up to 180 eV and appearance energies for CF₄, CCl₄ and CF₂Cl₂. In addition, the unimolecular dissociation processes CX₄⁺ → CX₃⁺ + X was investigated. (Author)

No abstract available

[en] We describe the development of a large-scale high-throughput application for discovery in materials science. Our point of departure is a computational framework for distributed multi-scale computation. We augment the original framework with a specialized module whose role is to route evaluation requests needed by the high-throughput application to a collection of available computational resources. We evaluate the feasibility and performance of the resulting high-throughput computational framework by carrying out a high-throughput study of battery solvents. Our results indicate that distributed multi-scale computing, by virtue of its adaptive nature, is particularly well-suited for building high-throughput applications. (paper)

[en] We present an efficient computational approach to perform real-space electronic structure calculations using an adaptive higher-order finite-element discretization of Kohn–Sham density-functional theory (DFT). To this end, we develop an a priori mesh-adaption technique to construct a close to optimal finite-element discretization of the problem. We further propose an efficient solution strategy for solving the discrete eigenvalue problem by using spectral finite-elements in conjunction with Gauss–Lobatto quadrature, and a Chebyshev acceleration technique for computing the occupied eigenspace. The proposed approach has been observed to provide a staggering 100–200-fold computational advantage over the solution of a generalized eigenvalue problem. Using the proposed solution procedure, we investigate the computational efficiency afforded by higher-order finite-element discretizations of the Kohn–Sham DFT problem. Our studies suggest that staggering computational savings—of the order of 1000-fold—relative to linear finite-elements can be realized, for both all-electron and local pseudopotential calculations, by using higher-order finite-element discretizations. On all the benchmark systems studied, we observe diminishing returns in computational savings beyond the sixth-order for accuracies commensurate with chemical accuracy, suggesting that the hexic spectral-element may be an optimal choice for the finite-element discretization of the Kohn–Sham DFT problem. A comparative study of the computational efficiency of the proposed higher-order finite-element discretizations suggests that the performance of finite-element basis is competing with the plane-wave discretization for non-periodic local pseudopotential calculations, and compares to the Gaussian basis for all-electron calculations to within an order of magnitude. Further, we demonstrate the capability of the proposed approach to compute the electronic structure of a metallic system containing 1688 atoms using modest computational resources, and good scalability of the present implementation up to 192 processors

[en] Applications such as grid-based real-space density functional theory (DFT) use the Poisson equation to compute electrostatics. However, the expected long tail of the electrostatic potential requires either the use of a large and costly outer domain or Dirichlet boundary conditions estimated via multipole expansion. We find that the oft-used single-center spherical multipole expansion is only appropriate for isotropic mesh domains such as spheres and cubes. In this work, we introduce a method suitable for high aspect ratio meshes whereby the charge density is partitioned into atomic domains and multipoles are computed for each domain. While this approach is moderately more expensive than a single-center expansion, it is numerically stable and still a small fraction of the overall cost of a DFT calculation. The net result is that when high aspect ratio systems are being studied, form-fitted meshes can now be used in lieu of cubic meshes to gain computational speedup.