563 research outputs found
Fourier Acceleration of Langevin Molecular Dynamics
Fourier acceleration has been successfully applied to the simulation of
lattice field theories for more than a decade. In this paper, we extend the
method to the dynamics of discrete particles moving in continuum. Although our
method is based on a mapping of the particles' dynamics to a regular grid so
that discrete Fourier transforms may be taken, it should be emphasized that the
introduction of the grid is a purely algorithmic device and that no smoothing,
coarse-graining or mean-field approximations are made. The method thus can be
applied to the equations of motion of molecular dynamics (MD), or its Langevin
or Brownian variants. For example, in Langevin MD simulations our acceleration
technique permits a straightforward spectral decomposition of forces so that
the long-wavelength modes are integrated with a longer time step, thereby
reducing the time required to reach equilibrium or to decorrelate the system in
equilibrium. Speedup factors of up to 30 are observed relative to pure
(unaccelerated) Langevin MD. As with acceleration of critical lattice models,
even further gains relative to the unaccelerated method are expected for larger
systems. Preliminary results for Fourier-accelerated molecular dynamics are
presented in order to illustrate the basic concepts. Possible extensions of the
method and further lines of research are discussed.Comment: 11 pages, two illustrations included using graphic
Approach to equilibrium for a class of random quantum models of infinite range
We consider random generalizations of a quantum model of infinite range
introduced by Emch and Radin. The generalization allows a neat extension from
the class of absolutely summable lattice potentials to the optimal class
of square summable potentials first considered by Khanin and Sinai and
generalised by van Enter and van Hemmen. The approach to equilibrium in the
case of a Gaussian distribution is proved to be faster than for a Bernoulli
distribution for both short-range and long-range lattice potentials. While
exponential decay to equilibrium is excluded in the nonrandom case, it is
proved to occur for both short and long range potentials for Gaussian
distributions, and for potentials of class in the Bernoulli case. Open
problems are discussed.Comment: 10 pages, no figures. This last version, to appear in J. Stat. Phys.,
corrects some minor errors and includes additional references and comments on
the relation to experiment
Lattice Sigma Models with Exact Supersymmetry
We show how to construct lattice sigma models in one, two and four dimensions
which exhibit an exact fermionic symmetry. These models are discretized and
{\it twisted} versions of conventional supersymmetric sigma models with N=2
supersymmetry. The fermionic symmetry corresponds to a scalar BRST charge built
from the original supercharges. The lattice theories possess local actions and
in many cases admit a Wilson term to suppress doubles. In the two and four
dimensional theorie s we show that these lattice theories are invariant under
additional discrete symmetries. We argue that the presence of these exact
symmetries ensures that no fine tuning is required to achieve N=2 supersymmetry
in the continuum limit. As a concrete example we show preliminary numerical
results from a simulation of the O(3) supersymmetric sigma model in two
dimensions.Comment: 23 pages, 3 figures, formalism generalized to allow for explicit
Wilson mass terms. New numerical results added. Version to be published in
JHE
Modeling parasite dynamics on farmed salmon for precautionary conservation management of wild salmon
Conservation management of wild fish may include fish health management in sympatric populations of domesticated fish in aquaculture. We developed a mathematical model for the population dynamics of parasitic sea lice (Lepeophtheirus salmonis) on domesticated populations of Atlantic salmon (Salmo salar) in the Broughton Archipelago region of British Columbia. The model was fit to a seven-year dataset of monthly sea louse counts on farms in the area to estimate population growth rates in relation to abiotic factors (temperature and salinity), local host density (measured as cohort surface area), and the use of a parasiticide, emamectin benzoate, on farms. We then used the model to evaluate management scenarios in relation to policy guidelines that seek to keep motile louse abundance below an average three per farmed salmon during the March-June juvenile wild Pacific salmon (Oncorhynchus spp.) migration. Abiotic factors mediated the duration of effectiveness of parasiticide treatments, and results suggest treatment of farmed salmon conducted in January or early February minimized average louse abundance per farmed salmon during the juvenile wild salmon migration. Adapting the management of parasites on farmed salmon according to migrations of wild salmon may therefore provide a precautionary approach to conserving wild salmon populations in salmon farming regions
Logarithmic two-loop corrections to the Lamb shift in hydrogen
Higher order logarithmic corrections to the
hydrogen Lamb shift are calculated. The results obtained show the two-loop
contribution has a very peculiar behavior, and significantly alter the
theoretical predictions for low lying S-states.Comment: 14 pages, including 2 figures, submitted to Phys. Rev. A, updated
with minor change
Recoil correction to the ground state energy of hydrogenlike atoms
The recoil correction to the ground state energy of hydrogenlike atoms is
calculated to all orders in \alpha Z in the range Z = 1-110. The nuclear size
corrections to the recoil effect are partially taken into account. In the case
of hydrogen, the relativistic recoil correction beyond the Salpeter
contribution and the nonrelativistic nuclear size correction to the recoil
effect, amounts to -7.2(2) kHz. The total recoil correction to the ground state
energy in hydrogenlike uranium (^{238}U^{91+}) constitutes 0.46 eV.Comment: 16 pages, 1 figure (eps), Latex, submitted to Phys.Rev.
Baryon Charge Radii and Quadrupole Moments in the 1/N_c Expansion: The 3-Flavor Case
We develop a straightforward method to compute charge radii and quadrupole
moments for baryons both with and without strangeness, when the number of QCD
color charges is N_c. The minimal assumption of the single-photon exchange
ansatz implies that only two operators are required to describe these baryon
observables. Our results are presented so that SU(3) flavor and isospin
symmetry breaking can be introduced according to any desired specification,
although we also present results obtained from two patterns suggested by the
quark model with gluon exchange interactions. The method also permits to
extract a number of model-independent relations; a sample is r^2_Lambda / r_n^2
= 3/(N_c+3), independent of SU(3) symmetry breaking.Comment: 30 pages, no figures, REVTeX
Nucleon Axial Form Factor from Lattice QCD
Results for the isovector axial form factors of the proton from a lattice QCD
calculation are presented for both point-split and local currents. They are
obtained on a quenched lattice at with Wilson
fermions for a range of quark masses from strange to charm. We determine the
finite lattice renormalization for both the local and point-split currents of
heavy quarks. Results extrapolated to the chiral limit show that the
dependence of the axial form factor agrees reasonably well with experiment. The
axial coupling constant calculated for the local and the point-split
currents is about 6\% and 12\% smaller than the experimental value
respectively.Comment: 8 pages, 5 figures (included in part 2), UK/93-0
Multiplicativity of completely bounded p-norms implies a new additivity result
We prove additivity of the minimal conditional entropy associated with a
quantum channel Phi, represented by a completely positive (CP),
trace-preserving map, when the infimum of S(gamma_{12}) - S(gamma_1) is
restricted to states of the form gamma_{12} = (I \ot Phi)(| psi >< psi |). We
show that this follows from multiplicativity of the completely bounded norm of
Phi considered as a map from L_1 -> L_p for L_p spaces defined by the Schatten
p-norm on matrices; we also give an independent proof based on entropy
inequalities. Several related multiplicativity results are discussed and
proved. In particular, we show that both the usual L_1 -> L_p norm of a CP map
and the corresponding completely bounded norm are achieved for positive
semi-definite matrices. Physical interpretations are considered, and a new
proof of strong subadditivity is presented.Comment: Final version for Commun. Math. Physics. Section 5.2 of previous
version deleted in view of the results in quant-ph/0601071 Other changes
mino
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