59,499 research outputs found
Ab initio quantum dynamics using coupled-cluster
The curse of dimensionality (COD) limits the current state-of-the-art {\it ab
initio} propagation methods for non-relativistic quantum mechanics to
relatively few particles. For stationary structure calculations, the
coupled-cluster (CC) method overcomes the COD in the sense that the method
scales polynomially with the number of particles while still being
size-consistent and extensive. We generalize the CC method to the time domain
while allowing the single-particle functions to vary in an adaptive fashion as
well, thereby creating a highly flexible, polynomially scaling approximation to
the time-dependent Schr\"odinger equation. The method inherits size-consistency
and extensivity from the CC method. The method is dubbed orbital-adaptive
time-dependent coupled-cluster (OATDCC), and is a hierarchy of approximations
to the now standard multi-configurational time-dependent Hartree method for
fermions. A numerical experiment is also given.Comment: 5 figure
Finite-size scaling in complex networks
A finite-size-scaling (FSS) theory is proposed for various models in complex
networks. In particular, we focus on the FSS exponent, which plays a crucial
role in analyzing numerical data for finite-size systems. Based on the
droplet-excitation (hyperscaling) argument, we conjecture the values of the FSS
exponents for the Ising model, the susceptible-infected-susceptible model, and
the contact process, all of which are confirmed reasonably well in numerical
simulations
On the Anomalous Temperature Distribution of the Intergalactic Medium in the NGC 3411 Group of Galaxies
We present XMM, Chandra and VLA observations of the USGC S152 group and its
central elliptical NGC 3411. Imaging of the group X-ray halo suggests it is
relaxed with little apparent structure. We investigate the temperature and
metal abundance structure of the group halo, and find that while the abundance
distribution is fairly typical, the temperature profile is highly unusual,
showing a hot inner core surrounded by a cool shell of gas with a radius of
\~20-40 kpc, at the center of the larger group halo. Spectral mapping confirms
an irregular ring of gas ~0.15 keV cooler than its surroundings. We estimate
the total mass, entropy and cooling time profiles within ~200 kpc, and find
that the cool shell contains ~9x10^9 Msun of gas. VLA observations at 1.4, 5
and 8 GHz reveal a relatively weak nuclear radio source, with a core radio
luminosity L_R=2.7x10^38 erg/s, and a diffuse component extended on scales of a
few arcseconds (or more). A lack of evidence for activity at optical or X-ray
wavelengths supports the conclusion that the central black hole is currently in
a quiescent state. We discuss possible mechanisms for the formation of
temperature features observed in the halo, including a previous period of AGN
activity, and settling of material stripped from the halo of one of the other
group member galaxies.Comment: 15 pages, 8 postscript figures, accepted for publication in ApJ
version with high resolution figures available at
http://hea-www.harvard.edu/~ejos/files/N3411_hires.pd
Universality Class of One-Dimensional Directed Sandpile Models
A general n-state directed `sandpile' model is introduced. The stationary
properties of the n-state model are derived for n < infty, and analytical
arguments based on a central limit theorem show that the model belongs to the
universality class of the totally asymmetric Oslo model, with a crossover to
uncorrelated branching process behavior for small system sizes. Hence, the
central limit theorem allows us to identify the existence of a large
universality class of one-dimensional directed sandpile models.Comment: 4 pages, 2 figure
Branching process approach for Boolean bipartite networks of metabolic reactions
The branching process (BP) approach has been successful in explaining the
avalanche dynamics in complex networks. However, its applications are mainly
focused on unipartite networks, in which all nodes are of the same type. Here,
motivated by a need to understand avalanche dynamics in metabolic networks, we
extend the BP approach to a particular bipartite network composed of Boolean
AND and OR logic gates. We reduce the bipartite network into a unipartite
network by integrating out OR gates, and obtain the effective branching ratio
for the remaining AND gates. Then the standard BP approach is applied to the
reduced network, and the avalanche size distribution is obtained. We test the
BP results with simulations on the model networks and two microbial metabolic
networks, demonstrating the usefulness of the BP approach
Forcing anomalous scaling on demographic fluctuations
We discuss the conditions under which a population of anomalously diffusing
individuals can be characterized by demographic fluctuations that are
anomalously scaling themselves. Two examples are provided in the case of
individuals migrating by Gaussian diffusion, and by a sequence of L\'evy
flights.Comment: 5 pages 2 figure
Progressive failure methodologies for predicting residual strength and life of laminated composites
Two progressive failure methodologies currently under development by the Mechanics of Materials Branch at NASA Langley Research Center are discussed. The damage tolerance/fail safety methodology developed by O'Brien is an engineering approach to ensuring adequate durability and damage tolerance by treating only delamination onset and the subsequent delamination accumulation through the laminate thickness. The continuum damage model developed by Allen and Harris employs continuum damage laws to predict laminate strength and life. The philosophy, mechanics framework, and current implementation status of each methodology are presented
HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000
red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample
of stars to derive the metallicity distribution function (MDF) for its halo
field stars, and comment on its chemical evolution history compared with both
larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending
from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers
a significant portion of this galaxy's halo. We find that the MDF is broad,
reaching a peak at [m/H] ~ -0.6. It may, in addition, have relatively few
stars more metal-rich than [m/H] = -0.3$, although interpretation of the
high-metallicity end of the MDF is limited by photometric completeness that
affects the detection of the reddest, most metal-rich stars. NGC 3377 appears
to have an enrichment history intermediate between those of normal dwarf
ellipticals and the much larger giants. As yet, we find no clear evidence that
the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy)
stars.Comment: 40 pages, 17 figure
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