6,013 research outputs found
Electron localisation in static and time-dependent one-dimensional model systems
Electron localization is the tendency of an electron in a many-body system to
exclude other electrons from its vicinity. Using a new natural measure of
localization based on the exact manyelectron wavefunction, we find that
localization can vary considerably between different ground-state systems, and
can also be strongly disrupted, as a function of time, when a system is driven
by an applied electric field. We use our new measure to assess the well-known
electron localization function (ELF), both in its approximate single-particle
form (often applied within density-functional theory) and its full
many-particle form. The full ELF always gives an excellent description of
localization, but the approximate ELF fails in time-dependent situations, even
when the exact Kohn-Sham orbitals are employed.Comment: 7 pages, 4 figure
Understanding thio-effects in simple phosphoryl systems : role of solvent effects and nucleophile charge.
Recent experimental work (J. Org. Chem., 2012, 77, 5829) demonstrated pronounced differences in measured thio-effects for the hydrolysis of (thio)phosphodichloridates by water and hydroxide nucleophiles. In the present work, we have performed detailed quantum chemical calculations of these reactions, with the aim of rationalizing the molecular bases for this discrimination. The calculations highlight the interplay between nucleophile charge and transition state solvation in SN2(P) mechanisms as the basis of these differences, rather than a change in mechanism
Determination of the stiffness of the nuclear symmetry energy from isospin diffusion
With an isospin- and momentum-dependent transport model, we find that the
degree of isospin diffusion in heavy ion collisions at intermediate energies is
affected by both the stiffness of the nuclear symmetry energy and the momentum
dependence of the nucleon potential. Using a momentum dependence derived from
the Gogny effective interaction, recent experimental data from NSCL/MSU on
isospin diffusion are shown to be consistent with a nuclear symmetry energy
given by at
subnormal densities. This leads to a significantly constrained value of about
-550 MeV for the isospin-dependent part of the isobaric incompressibility of
isospin asymmetric nuclear matter.Comment: 4 pages, 4 figures, 1 table, revised version, to appear in PR
Spherical structures on torus knots and links
The present paper considers two infinite families of cone-manifolds endowed
with spherical metric. The singular strata is either the torus knot or the torus link . Domains of existence for a
spherical metric are found in terms of cone angles and volume formul{\ae} are
presented.Comment: 17 pages, 5 figures; typo
Reconstructing the global topology of the universe from the cosmic microwave background
If the universe is multiply-connected and sufficiently small, then the last
scattering surface wraps around the universe and intersects itself. Each circle
of intersection appears as two distinct circles on the microwave sky. The
present article shows how to use the matched circles to explicitly reconstruct
the global topology of space.Comment: 6 pages, 2 figures, IOP format. To be published in the proceedings of
the Cleveland Cosmology and Topology Workshop 17-19 Oct 1997. Submitted to
Class. Quant. Gra
The maximal tubes under the deformations of a class of 3-dimensional hyperbolic cone-manifolds
Recently, Hodgson and Kerckhoff found a small bound on Dehn surgered
3-manifolds from hyperbolic knots not admitting hyperbolic structures using
deformations of hyperbolic cone-manifolds. They asked whether the area
normalized meridian length squared of maximal tubular neighborhoods of the
singular locus of the cone-manifold is decreasing and that summed with the cone
angle squared is increasing as we deform the cone-angles. We confirm this near
0 cone-angles for an infinite family of hyperbolic cone-manifolds obtained by
Dehn surgeries along the Whitehead link complements. The basic method is based
on explicit holonomy computations using the A-polynomials and finding the
maximal tubes. One of the key tool is the Taylor expression of a geometric
component of the zero set of the A-polynomial in terms of the cone-angles. We
also show a sequence of Taylor expressions for Dehn surgered manifolds
converges to one for the limit hyperbolic manifold.Comment: 27 pages, 10 figure
Effects of Management Strategies on Seed Production and Seedling Recruitment in Birdsfoot Trefoil-White Clover Mixtures
The effects of grazing management on seed production, seed bank size and seedling emergence patterns of Trifolium repens cv. Zapicán (WC) and Lotus corniculatus cv. San Gabriel (BFT) oversown swards were evaluated. A complete randomized block design with 4 replicate blocks was used, in which 4 grazing strategies (grazing all year (SI), summer spelling for seed production (S2), winter rest plus summer spelling (S3) and autumn rest plus summer spelling (S4)), were combined with two defoliation severalties (4 and 10 cm height post-grazing residuals). Plots of 110 m2 were grazed monthly by sheep. Seed production, soil seed bank and seedling emergence were monitored from April 1998 to August 1999. In both species, summer spelling for seed production improved seed yield, especially in BFT. Severe defoliation (4 cm) reduced seed inputs drastically (46% in BFT and 64% in WC). 1000 seed weight was only affected by defoliation severity in WC (0.544 and 0.562 g for 4 and 10 cm height respectively). Potential seedling emergence, between June and December from soil seed bank, was 44 and 35% in BFT and WC, respectively. Seedling emergence of Lotus corniculatus increased under high seed production levels (S3), and also it was improved under intensive grazing during autumn and winter. There were no effects on Trifolium repens seedling emergence. Soil seed bank can preserve seedling recruitment rates in the short term, but maintenance of species balance will depend on seeding spelling management
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