8,542 research outputs found
Generalized valence bond wave functions in quantum Monte Carlo
We present a technique for using quantum Monte Carlo (QMC) to obtain high quality energy differences. We use generalized valence bond (GVB) wave functions, for an intuitive approach to capturing the important sources of static correlation, without needing to optimize the orbitals with QMC. Using our modifications to Walker branching and Jastrows, we can then reliably use diffusion quantum Monte Carlo to add in all the dynamic correlation. This simple approach is easily accurate to within a few tenths of a kcal/mol for a variety of problems, which we demonstrate for the adiabatic singlet-triplet splitting in methylene, the vertical and adiabatic singlet-triplet splitting in ethylene, 2+2 cycloaddition, and Be_2 bond breaking
Collective-coupling analysis of spectra of mass-7 isobars: ^7He, ^7Li, ^7Be, ^7B
A nucleon-nucleus interaction model has been applied to ascertain the
underlying character of the negative-parity spectra of four isobars of mass
seven, from neutron-- to proton--emitter driplines. With one single nuclear
potential defined by a simple coupled-channel model, a multichannel algebraic
scattering approach (MCAS) has been used to determine the bound and resonant
spectra of the four nuclides, of which ^7He and ^7B are particle unstable.
Incorporation of Pauli blocking in the model enables a description of all known
spin-parity states of the mass-7 isobars. We have also obtained spectra of
similar quality by using a large space no-core shell model. Additionally, we
have studied ^7Li and ^7Be using a dicluster model. We have found a
dicluster-model potential that can reproduce the lowest four states of the two
nuclei, as well as the relevant low-energy elastic scattering cross sections.
But, with this model, the rest of the energy spectra cannot be obtained.Comment: Extended version published in Phys. Rev. C, 16 pages and 5 figure
Is friction responsible for the reduction of fusion rates far below the Coulomb barrier?
The fusion of two interacting heavy ions traditionally has been interpreted
in terms of the penetration of the projectile into the target. Observed rates
well below the Coulomb barrier are considerably lower than estimates obtained
from penetration factors. One approach in the analysis of the data invokes
coupling to non-elastic channels in the scattering as the source of the
depletion. Another is to analyze those data in terms of tunneling in
semi-classical models, with the observed depletion being taken as evidence of a
``friction'' under the barrier. A complementary approach is to consider such
tunneling in terms of a fully quantal model. We investigate tunneling with both
one-dimensional and three-dimensional models in a fully quantal approach to
investigate possible sources for such a friction. We find that the observed
phenomenon may not be explained by friction. However, we find that under
certain conditions tunneling may be enhanced or diminished by up to 50%, which
finds analogy with observation, without the invocation of a friction under the
barrier.Comment: 18 pages, 15 figures embedde
Non-localities in nucleon-nucleus potentials
Two causes of non-locality inherent in nucleon-nucleus scattering are
considered. They are the results of two-nucleon antisymmetry of the projectile
with each nucleon in the nucleus and the dynamic polarization potential
representation of channel coupling. For energies MeV, a
g-folding model of the optical potential is used to show the influence of the
knock-out process that is a result of the two-nucleon antisymmetry. To explore
the dynamic polarization potential caused by channel coupling, a multichannel
algebraic scattering model has been used for low-energy scattering.Comment: 12 pages, 11 figures, submitted to EPJ
Stakeholder and Constraint-Driven Innovation of a Novel, Lever-Propelled, All-Terrain Wheelchair
The Leveraged Freedom Chair (LFC) is a low-cost, all-terrain, lever-propelled wheelchair designed primarily for use in developing countries. LFC technology was conceived because 70 percent of wheelchair users in these markets live in rural areas and no currently available mobility aid enables them to travel long distances on rough terrain and maneuver in tight, indoor confines. Because developing world markets impose constraints on cost, durability, and performance, a novel solution was required to satisfy stakeholder requirements. The key innovation behind the LFC is its single speed, variable mechanical advantage lever drivetrain. The user effectively changes gear by shifting his hands along the levers; grasping near the ends increases torque, while grasping near the pivots enables a larger angular displacement with every stroke, which increases speed. The drivetrain is made from low-cost bicycle parts found throughout the developing world, which enables the LFC to be sold for $200 and be repairable anywhere.
During three user trials in East Africa, Guatemala, and India, stakeholder feedback was used to refine the chair between trials, resulting in a device 9.1 kg (20 lbs) lighter, 8.9 cm (3.5 in) narrower, and with a center of gravity 12.7 cm (5 in) lower than the first iteration. Survey data substantiated increases in performance after successive iterations. Quantitative biomechanical performance data were also measured during the Guatemala and India trials, which showed the LFC to be 76 percent faster and 41 percent more efficient during a common daily commute, and able to produce 53 percent higher peak propulsion force compared to conventional, pushrim-propelled wheelchairs. The LFC offers comparable performance at less than one-twentieth the cost of off road wheelchairs available in the rich world. Stakeholder feedback and the highly-constrained environment for which the LFC was created drove the technology towards a novel, innovative solution that offers a competitive advantage in both developing and developed markets. The paper concludes with a description of how the LFC is a “constraint-driven innovation.” This idea ties together the theories of “disruptive innovation” and “reverse innovation,” and may be used as a design tool for engineers striving to create technologies that have global impact.Singapore University of Technology and DesignInter-American Development BankNational Collegiate Inventors and Innovators AllianceD-Lab (Massachusetts Institute of Technology)Clinton Global InitiativeMassachusetts Institute of Technology. Office of the Dean for Graduate Education (Hugh Hampton Young Memorial Fellowship)Massachusetts Institute of Technology. Department of Mechanical EngineeringMassachusetts Institute of Technology. Public Service CenterMassachusetts Institute of Technology. Edgerton CenterMassachusetts Institute of Technology. Undergraduate Research Opportunities ProgramCalifornia Environmental Protection Agency. Air Resources Boar
Eikonal profile functions and amplitudes for and scattering
The eikonal profile function obtained from the Model of the Stochastic
Vacuum is parametrized in a form suitable for comparison with experiment. The
amplitude and the extended profile function (including imaginary and real
parts) are determined directly from the complete pp and p elastic
scattering data at high energies. Full and accurate representation of the data
is presented, with smooth energy dependence of all parameters. The changes
needed in the original profile function required for description of scattering
beyond the forward direction are described.Comment: Latex, 28 pages and 16 figure
Role of the Pauli principle in collective-model coupled-channels calculations
A multi-channel algebraic scattering theory, to find solutions of
coupled-channel scattering problems with interactions determined by collective
models, has been structured to ensure that the Pauli principle is not violated.
By tracking the results in the zero coupling limit, a correct interpretation of
the sub-threshold and resonant spectra of the compound system can be made. As
an example, the neutron-12C system is studied defining properties of 13C to 10
MeV excitation. Accounting for the Pauli principle in collective
coupled-channels models is crucial to the outcome.Comment: 4 pages, 1 figure, version appearing in Phys. Rev. Let
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