13,387 research outputs found
Fixed-parameter tractability of multicut parameterized by the size of the cutset
Given an undirected graph , a collection of
pairs of vertices, and an integer , the Edge Multicut problem ask if there
is a set of at most edges such that the removal of disconnects
every from the corresponding . Vertex Multicut is the analogous
problem where is a set of at most vertices. Our main result is that
both problems can be solved in time , i.e.,
fixed-parameter tractable parameterized by the size of the cutset in the
solution. By contrast, it is unlikely that an algorithm with running time of
the form exists for the directed version of the problem, as
we show it to be W[1]-hard parameterized by the size of the cutset
Quantum Fluctuations Driven Orientational Disordering: A Finite-Size Scaling Study
The orientational ordering transition is investigated in the quantum
generalization of the anisotropic-planar-rotor model in the low temperature
regime. The phase diagram of the model is first analyzed within the mean-field
approximation. This predicts at a phase transition from the ordered to
the disordered state when the strength of quantum fluctuations, characterized
by the rotational constant , exceeds a critical value . As a function of temperature, mean-field theory predicts a range of
values of where the system develops long-range order upon cooling, but
enters again into a disordered state at sufficiently low temperatures
(reentrance). The model is further studied by means of path integral Monte
Carlo simulations in combination with finite-size scaling techniques,
concentrating on the region of parameter space where reentrance is predicted to
occur. The phase diagram determined from the simulations does not seem to
exhibit reentrant behavior; at intermediate temperatures a pronounced increase
of short-range order is observed rather than a genuine long-range order.Comment: 27 pages, 8 figures, RevTe
Time-reversible Born-Oppenheimer molecular dynamics
We present a time-reversible Born-Oppenheimer molecular dynamics scheme,
based on self-consistent Hartree-Fock or density functional theory, where both
the nuclear and the electronic degrees of freedom are propagated in time. We
show how a time-reversible adiabatic propagation of the electronic degrees of
freedom is possible despite the non-linearity and incompleteness of the
self-consistent field procedure. Time-reversal symmetry excludes a systematic
long-term energy drift for a microcanonical ensemble and the number of
self-consistency cycles can be kept low (often only 2-4 cycles per nuclear time
step) thanks to a good initial guess given by the adiabatic propagation of the
electronic degrees of freedom. The time-reversible Born-Oppenheimer molecular
dynamics scheme therefore combines a low computational cost with a physically
correct time-reversible representation of the dynamics, which preserves a
detailed balance between propagation forwards and backwards in time.Comment: 4 pages, 4 figure
Comparison of Josephson vortex flow transistors with different gate line configurations
We performed numerical simulations and experiments on Josephson vortex flow
transistors based on parallel arrays of YBa2Cu3O(7-x) grain boundary junctions
with a cross gate-line allowing to operate the same devices in two different
modes named Josephson fluxon transistor (JFT) and Josephson fluxon-antifluxon
transistor (JFAT). The simulations yield a general expression for the current
gain vs. number of junctions and normalized loop inductance and predict higher
current gain for the JFAT. The experiments are in good agreement with
simulations and show improved coupling between gate line and junctions for the
JFAT as compared to the JFT.Comment: 3 pages, 6 figures, accept. for publication in Appl. Phys. Let
A tight lower bound for steiner orientation
In the STEINER ORIENTATION problem, the input is a mixed graph G (it has both directed and undirected edges) and a set of k terminal pairs T. The question is whether we can orient the undirected edges in a way such that there is a directed s⇝t path for each terminal pair (s,t)∈T. Arkin and Hassin [DAM’02] showed that the STEINER ORIENTATION problem is NP-complete. They also gave a polynomial time algorithm for the special case when k=2
.
From the viewpoint of exact algorithms, Cygan, Kortsarz and Nutov [ESA’12, SIDMA’13] designed an XP algorithm running in nO(k) time for all k≥1. Pilipczuk and Wahlström [SODA ’16] showed that the STEINER ORIENTATION problem is W[1]-hard parameterized by k. As a byproduct of their reduction, they were able to show that under the Exponential Time Hypothesis (ETH) of Impagliazzo, Paturi and Zane [JCSS’01] the STEINER ORIENTATION problem does not admit an f(k)⋅no(k/logk) algorithm for any computable function f. That is, the nO(k) algorithm of Cygan et al. is almost optimal.
In this paper, we give a short and easy proof that the nO(k) algorithm of Cygan et al. is asymptotically optimal, even if the input graph has genus 1. Formally, we show that the STEINER ORIENTATION problem is W[1]-hard parameterized by the number k of terminal pairs, and, under ETH, cannot be solved in f(k)⋅no(k) time for any function f even if the underlying undirected graph has genus 1. We give a reduction from the GRID TILING problem which has turned out to be very useful in proving W[1]-hardness of several problems on planar graphs. As a result of our work, the main remaining open question is whether STEINER ORIENTATION admits the “square-root phenomenon” on planar graphs (graphs with genus 0): can one obtain an algorithm running in time f(k)⋅nO(k√) for PLANAR STEINER ORIENTATION, or does the lower bound of f(k)⋅no(k) also translate to planar graphs
Hydrogen Absorption Properties of Metal-Ethylene Complexes
Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a
single ethylene molecule can form stable complexes with light transition metals
(TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and
14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to
include a large number of other metals and different isomeric structures. We
obtained interesting results for light metals such as Li. The ethylene molecule
is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which
then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2
and absorption capacity of 16 wt %, a record high value reported so far. The
stability of the proposed metal-ethylene complexes was tested by extensive
calculations such as normal-mode analysis, finite temperature first-principles
molecular dynamics (MD) simulations, and reaction path calculations. The phonon
and MD simulations indicate that the proposed structures are stable up to 500
K. The reaction path calculations indicate about 1 eV activation barrier for
the TM2-ethylene complex to transform into a possible lower energy
configuration where the ethylene molecule is dissociated. Importantly, no
matter which isometric configuration the TM2-ethylene complex possesses, the TM
atoms are able to bind multiple hydrogen molecules with suitable binding energy
for room temperature storage. These results suggest that co-deposition of
ethylene with a suitable precursor of TM or Li into nanopores of light-weight
host materials may be a very promising route to discovering new materials with
high-capacity hydrogen absorption properties
A Comprehensive Approach to API RP 752 and 753 Building Siting Studies
PresentationFacility siting studies are an important part of process safety, and are required for facilities that fall under OSHA’s PSM program. Facility siting is frequently interpreted as performing a building siting study which adheres to the guidance given in API RP 752. Facility siting may also consider siting of temporary or portable buildings based on the guidance in API RP 753. While both API RP 752 and API RP 753 provide a framework and some guidance for performing building siting studies, they do not provide detailed methodologies or provide guidance on performing a detailed analysis. As a result many building siting studies are inconsistent in their overall approach, or in the way they address hazards. Due to the recent scrutiny applied to building siting studies, more attention has been given to provide evaluations which correctly describe the range of hazards that may affect an occupied building at a petrochemical facility. This paper outlines a comprehensive methodology for performing building siting studies at such facilities. The methodology addresses the applicable hazards and the available tools by which the potential impacts to building occupants can be evaluated
A New Look at Release Event Frequencies
PresentationWithin the context of a quantitative risk analysis (QRA), the two main constituents used to describe petrochemical risks are, and have always been, consequence and probability. The consequences of hazardous material accidents are easy to apprehend – if a hazard is realized it can injure people or cause fatalities, damage equipment or other assets, or cause environmental damage. Frequencies for these consequences, on the other hand, are not as easy to understand. Process safety professionals develop event frequencies by evaluating historical data and calculating incident rates, which represent, in the QRA context, how often a release of a hazardous material has occurred. Incident rates are further modified by probabilities for various hole sizes, release orientations, weather conditions, ignition timing, and other factors, to arrive at unique event probabilities that are applied in the QRA. This paper describes the development of incident rates from historical database information for various equipment types, as well as defining a methodology for assigning hole size probabilities from the same data, such that a hole size distribution can be assigned within each QRA study. The combination of total incident rates and a hole size distribution relationship can then serve as a foundation within the frequency side of many QRA studies
Nuclear quantum effects in solids using a colored-noise thermostat
We present a method, based on a non-Markovian Langevin equation, to include
quantum corrections to the classical dynamics of ions in a quasi-harmonic
system. By properly fitting the correlation function of the noise, one can vary
the fluctuations in positions and momenta as a function of the vibrational
frequency, and fit them so as to reproduce the quantum-mechanical behavior,
with minimal a priori knowledge of the details of the system. We discuss the
application of the thermostat to diamond and to ice Ih. We find that results in
agreement with path-integral molecular dynamics can be obtained using only a
fraction of the computational effort.Comment: submitted for publicatio
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