21,244 research outputs found
Dynamical Multiple-Timestepping Methods for Overcoming the Half-Period Time Step Barrier
Current molecular dynamic simulations of biomolecules using multiple time
steps to update the slowingly changing force are hampered by an instability
occuring at time step equal to half the period of the fastest vibrating mode.
This has became a critical barrier preventing the long time simulation of
biomolecular dynamics. Attemps to tame this instability by altering the slowly
changing force and efforts to damp out this instability by Langevin dynamics do
not address the fundamental cause of this instability. In this work, we trace
the instability to the non-analytic character of the underlying spectrum and
show that a correct splitting of the Hamiltonian, which render the spectrum
analytic, restores stability. The resulting Hamiltonian dictates that in
additional to updating the momentum due to the slowly changing force, one must
also update the position with a modified mass. Thus multiple-timestepping must
be done dynamically.Comment: 10 pages, 2 figures, submitted to J. Chem. Phy
Signatures of Energy Flux in Particle Production: A Black Hole Birth Cry and Death Gasp
It is recently argued that if the Hawking radiation process is unitary, then
a black hole's mass cannot be monotonically decreasing. We examine the time
dependent particle count and negative energy flux in the non-trivial conformal
vacuum via the moving mirror approach. A new, exactly unitary solution is
presented which emits a characteristic above-thermal positive energy burst, a
thermal plateau, and negative energy flux. It is found that the characteristic
positive energy flare and thermal plateau is observed in the particle outflow.
However, the results of time dependent particle production show no overt
indication of negative energy flux. Therefore, a black hole's birth cry is
detectable by asymptotic observers via particle count, whereas its death gasp
is not.Comment: 12 pages, 6 figure
Exact evolution of time-reversible symplectic integrators and their phase error for the harmonic oscillator
The evolution of any factorized time-reversible symplectic integrators, when
applied to the harmonic oscillator, can be exactly solved in a closed form. The
resulting modified Hamiltonians demonstrate the convergence of the Lie series
expansions. They are also less distorted than modified Hamiltonian of
non-reversible algorithms. The analytical form for the modified angular
frequency can be used to assess the phase error of any time-reversible
algorithm.Comment: Submitted to Phys. Lett. A, Six Pages two Column
Fourth Order Gradient Symplectic Integrator Methods for Solving the Time-Dependent Schr\"odinger Equation
We show that the method of splitting the operator
to fourth order with purely positive coefficients produces excellent algorithms
for solving the time-dependent Schr\"odinger equation. These algorithms require
knowing the potential and the gradient of the potential. One 4th order
algorithm only requires four Fast Fourier Transformations per iteration. In a
one dimensional scattering problem, the 4th order error coefficients of these
new algorithms are roughly 500 times smaller than fourth order algorithms with
negative coefficient, such as those based on the traditional Ruth-Forest
symplectic integrator. These algorithms can produce converged results of
conventional second or fourth order algorithms using time steps 5 to 10 times
as large. Iterating these positive coefficient algorithms to 6th order also
produced better converged algorithms than iterating the Ruth-Forest algorithm
to 6th order or using Yoshida's 6th order algorithm A directly.Comment: 11 pages, 2 figures, submitted to J. Chem. Phy
LAMMR world data base documentation support and demonstrations
The primary purpose of the World Surface Map is to provide the LAMMR subsystem with world surface type classifications that are used to set up LAMMR LEVEL II process control. This data base will be accessed solely by the LAMMR subsystem. The SCATT and ALT subsystems will access the data base indirectly through the T sub b (Brightness Temperature) Data Bank, where the surface types were updated from a priori to current classification, and where the surface types were organized on an orbital subtrack basis. The single most important factor in the design of the World Surface Maps is the ease of access to the information while the complexity of generating these maps is of lesser importance because their generation is a one-time, off-line process. The World Surface Map provides storage of information with a resolution of 7 km necessary to set flags concerning the earth's features with a different set of maps for each month of the year
Context sensitive formulations of antenna pattern correction and side lobe compensation for NOSS/LAMMR real time processing
Large antenna multi-channel microwave radiometer (LAMMR) software specifications were written for LAMMR ground processing. There is a need to determine more computationally-efficient antenna temperature correction methods in compensating side lobe contributions especially near continents, islands and weather fronts. One of the major conclusions was that the antenna pattern corrections (APC) processes did not accomplish the implied goals of compensating for the antenna side lobe influences on brightness temperature. A-priori knowledge of land/water locations was shown to be needed and had to be incorporated in a context sensitive APC process if the artifacts caused by land presence is to be avoided. The high temperatures in land regions can severely bias the lower ocean response
Evolution of stars with suppressed core convection
Stellar evolution on the upper main sequence was computed for models of stars with cores assumed to be in radiative equilibrium, up to the point of central helium ignition. The role of the Schonberg-Chandrasekhar limit for an isothermal core is found to be critical for the evolutionary tracks. Observational data are used to rule out the hypothesis of evolution with radiative cores (in upper main-sequence stars) and, by implication, of magnetic fields that are sufficiently strong to have suppressed the core convention
Noise removal in multichannel images
A adaptive filtering method, the Windrow-Hoff algorithm, for enhancing multichannel signals against aditive noise was investigated. It removes noise for multichannel images containing correlated signal compoments but uncorrelated noise components. Its potential application is the enhancement of multichannel microwave satellite images as a preprocessing step for the extraction of geophysical parameters
Nonmetallic impurities improve mechanical properties of vapor-deposited tungsten
Mechanical properties of vapor deposited tungsten are improved by selective incorporation of various nonmetallic impurities. Addition of trace quantities of carbon, nitrogen, or oxygen can significantly increase both low and high temperature yield strength without greatly affecting ductile-to-brittle transition temperature
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