3,527 research outputs found
Comment to: "Noncovalent functionalization of carbon nanotubes by aromatic organic molecules" [ Appl. Phys. Lett. 82, 3746 (2003) ]
The weak-chemisorption/charge-transfer picture for adsorption of aromatic
molecules over carbon nanotubes, proposed in the commented paper, is
criticized.Comment: 1.5 pages, 1 figure, submitted to AP
Positional disorder in ammonia borane at ambient conditions
We solve a long-standing experimental discrepancy of NHBH, which---as
a molecule---has a threefold rotational axis, but in its crystallized form at
room temperature shows a fourfold symmetry about the same axis, creating a
geometric incompatibility. To explain this peculiar experimental result, we
study the dynamics of this system with ab initio Car-Parrinello molecular
dynamics and nudged-elastic-band simulations. We find that rotations, rather
than spatial static disorder, at angular velocities of 2 rev/ps---a time scale
too small to be resolved by standard experimental techniques---are responsible
for the fourfold symmetry
Strengthening governance of social safety nets in East Asia
Several East Asian countries, in the aftermath of the global financial crisis, are considering an expansion of their social safety net programs. In many cases, existing delivery mechanisms for social assistance in the region tend to be basic, in line with the small size of programs. In a context of coverage expansion and proliferation of new programs, the risk of creating increasingly complex systems characterized by cross-incentives is high. Lack of coordination, ambiguous criteria for identifying and selecting beneficiaries, low administrative capacity, lack of transparency and limited beneficiary participation pose risks for program effectiveness and can decrease accountability. Good governance can improve program outcomes through effective program coordination, stronger accountability arrangements, provider incentives and greater transparency and participation. This paper proposes an analytical framework to systematically identify governance risks and constraints which, if removed, could improve the outcomes of modern social assistance programs.Health Monitoring&Evaluation,Safety Nets and Transfers,National Governance,Governance Indicators,Poverty Monitoring&Analysis
First-principle molecular dynamics with ultrasoft pseudopotentials: parallel implementation and application to extended bio-inorganic system
We present a plane-wave ultrasoft pseudopotential implementation of
first-principle molecular dynamics, which is well suited to model large
molecular systems containing transition metal centers. We describe an efficient
strategy for parallelization that includes special features to deal with the
augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We
also discuss a simple approach to model molecular systems with a net charge
and/or large dipole/quadrupole moments. We present test applications to
manganese and iron porphyrins representative of a large class of biologically
relevant metallorganic systems. Our results show that accurate
Density-Functional Theory calculations on systems with several hundred atoms
are feasible with access to moderate computational resources.Comment: 29 pages, 4 Postscript figures, revtex
Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}
We study the unconventional insulating state in A_4C_{60} with a variety of
approaches, including density functional calculations and dynamical mean-field
theory. While the former predicts a metallic state, in disagreement with
experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In
that state, conduction between molecules is blocked by on-site Coulomb
repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and
important excitations (such as optical and spin gap) should be essentially
intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare
well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure
Software for quantum simulations of tomorrow
1noScientific software allows to transform theories and methods into numerical results and physical insight. This is especially true for the study of realistic models of materials, that heavily rely on first-principle computer simulations. This article describes the problems encountered in the development and maintenance of scientific software and examines the possible solutions. The ongoing work towards better software for present and future simulations of materials, performed with the Quantum ESPRESSO distribution in the framework of the EU-supported MaX Centre of Excellence, is introduced.openopenPaolo GiannozziGiannozzi, Paol
Redox functionality mediated by adsorbed oxygen on a Pd-oxide film over a Pd(100) thin structure: A first-principles study
Stable oxygen sites on a PdO film over a Pd(100) thin structures with a
(sqrt{5} times sqrt{5}) R27^circ surface-unit cell are determined using the
first-principles electronic structure calculations with the generalized
gradient approximation. The adsorbed monatomic oxygen goes to a site bridging
two 2-fold-coordinated Pd atoms or to a site bridging a 2-fold-coordinated Pd
atom and a 4-fold-coordinated Pd atom. Estimated reaction energies of CO
oxidation by reduction of the oxidized PdO film and N_2O reduction mediated by
oxidation of the PdO film are exothermic. Motion of the adsorbed oxygen atom
between the two stable sites is evaluated using the nudged elastic band method,
where an energy barrier for a translational motion of the adsorbed oxygen may
become sim 0.45 eV, which is low enough to allow fluxionality of the surface
oxygen at high temperatures. The oxygen fluxionality is allowed by existence of
2-fold-coordinated Pd atoms on the PdO film, whose local structure has
similarity to that of Pd catalysts for the Suzuki-Miyaura cross coupling.
Although NO_x (including NO_2 and NO) reduction is not always catalyzed only by
the PdO film, we conclude that there may happen continual redox reactions
mediated by oxygen-adsorbed PdO films over a Pd surface structure, when the
influx of NO_x and CO continues, and when the reaction cycle is kept on a
well-designed oxygen surface.Comment: 15 pages, 6 figures, submitted to J. Phys. Condens. Matte
Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on CuN/Cu(100) surface
We study the effects of the exchange interaction between an adsorbed magnetic
atom with easy-axis magnetic anisotropy and the conduction-band electrons from
the substrate. We model the system using an anisotropic Kondo model and we
compute the impurity spectral function which is related to the differential
conductance (dI/dV) spectra measured using a scanning tunneling microscope. To
make contact with the known experimental results for iron atoms on the
CuN/Cu(100) surface [Hirjibehedin et al., Science {\bf 317}, 1199 (2007)], we
calculated the spectral functions in the presence of an external magnetic field
of varying strength applied along all three spatial directions. It is possible
to establish an upper bound on the coupling constant J: in the range of the
magnetic fields for which the experimental results are currently known (up to
7T), the low-energy features in the calculated spectra agree well with the
measured dI/dV spectra if the exchange coupling constant J is at most half as
large as that for cobalt atoms on the same surface. We show that for even
higher magnetic field (between 8 and 9T) applied along the ``hollow
direction'', the impurity energy states cross, giving rise to a Kondo effect
which takes the form of a zero-bias resonance. The paper introduces an approach
for calculating the expectation values of global spin operators and all
components of the impurity magnetic susceptibility tensor in numerical
renormalization group (NRG) calculations with no spin symmetry. An appendix
contains a density-functional-theory (DFT) study of the Cu and Fe adsorbates on
CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies,
occupancies, centers, and spreads by calculating the maximally localized
Wannier orbitals of the adsorbates.Comment: 18 pages, 7 figure
Large-scale computing with Quantum ESPRESSO
This paper gives a short introduction to Quantum ESPRESSO: a
distribution of software for atomistic simulations in condensed-matter physics, chemical physics, materials science, and to its usage in large-scale parallel computing
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