25,395 research outputs found
Sampling in the multicanonical ensemble: Small He clusters in W
We carry out generalized-ensemble molecular dynamics simulations of the
formation of small Helium (He) clusters in bulk Tungsten (W), a process of
practical relevance for fusion energy production. We calculate formation free
energies of small Helium clusters at temperatures up to the melting point of W,
encompassing the whole range of interest for fusion-energy production. From
this, parameters like cluster break-up or formation rates can be calculated,
which help to refine models of microstructure evolution in He-irradiated
Tungsten.Comment: 27th Annual CSP Workshop on Recent Developments in Computer
Simulation Studies in Condensed Matter Physics, Athens, GA, 201
Generalization of the concepts of seniority number and ionicity
We present generalized versions of the concepts of seniority number and
ionicity. These generalized numbers count respectively the partially occupied
and fully occupied shells for any partition of the orbital space into shells.
The Hermitian operators whose eigenspaces correspond to wave functions of
definite generalized seniority or ionicity values are introduced. The
generalized seniority numbers (GSNs) afford to establish refined hierarchies of
configuration interaction (CI) spaces within those of fixed ordinary seniority.
Such a hierarchy is illustrated on the buckminsterfullerene molecule
Incidental Catch of Marine Mammals by Foreign and Joint Venture Trawl Vessels in the U.S. EEZ of the North Pacific, 1973-88
During 1973-88, 3,661 marine mammals of 17 species were reported as incidental catch by U.S. fishery observers aboard foreign and joint venture trawl vessels in the U.S.
Exclusive Economic Zone in the North Pacific Ocean and the Bering Sea. Northern sea lions (Eumetopias jubatus) accounted for 90% of the reported incidental mortality in the Gulf of Alaska and eastern Bering Sea. Nearly half of these sea lions were taken in trawl nets in the Shelikof Strait, Alaska, joint venture fishery during 1982-84. However, high incidental mortality rates (>25 sea lions per 10,000 metric tons of groundfish catch) also occurred in the foreign fisheries near Kodiak Island and in the Aleutian Islands area in earlier years. Estimated annual mortality of incidentally caught northern sea lions in
Alaska declined from 1,000 to 2,000 animals per year during the early 1970s and 1982 to fewer than 100 animals in 1988. In the Bering Sea most sea lions incidentally caught were
males, while in the Gulf of Alaska females were more frequently caught. Females may also have been dominant in the incidental catch of sea lions in the Aleutian Islands area, but age and sex composition data are limited. Incidental mortality of adult female sea lions by foreign trawl fisheries in these areas could have partially contributed to the reported declines in northern sea lion populations in Alaska during the 1970s, but it cannot alone
account for the present decline in population size. (PDF file contains 64 pages.
Self-optimized construction of transition rate matrices from accelerated atomistic simulations with Bayesian uncertainty quantification
A massively parallel method to build large transition rate matrices from
temperature accelerated molecular dynamics trajectories is presented. Bayesian
Markov model analysis is used to estimate the expected residence time in the
known state space, providing crucial uncertainty quantification for higher
scale simulation schemes such as kinetic Monte Carlo or cluster dynamics. The
estimators are additionally used to optimize where exploration is performed and
the degree of temperature ac- celeration on the fly, giving an autonomous,
optimal procedure to explore the state space of complex systems. The method is
tested against exactly solvable models and used to explore the dynamics of C15
interstitial defects in iron. Our uncertainty quantification scheme allows for
accurate modeling of the evolution of these defects over timescales of several
seconds.Comment: 14 pages, 7 figure
Extending the scope of models for large-scale structure formation in the Universe
We propose a phenomenological generalization of the models of large-scale
structure formation in the Universe by gravitational instability in two ways:
we include pressure forces to model multi-streaming, and noise to model
fluctuations due to neglected short-scale physical processes. We show that
pressure gives rise to a viscous-like force of the same character as that one
introduced in the ``adhesion model'', while noise leads to a roughening of the
density field yielding a scaling behavior of its correlations.Comment: matches published version in A&A, incl. 3 figure
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