2,475 research outputs found
Polynomial scaling approximations and dynamic correlation corrections to doubly occupied configuration interaction wave functions
A class of polynomial scaling methods that approximate Doubly Occupied Configuration Interaction (DOCI) wave functions and improve the description of dynamic correlation is introduced. The accuracy of the resulting wave functions is analysed by comparing energies and studying the overlap between the newly developed methods and full configuration interaction wave functions, showing that a low energy does not necessarily entail a good approximation of the exact wave function. Due to the dependence of DOCI wave functions on the single-particle basis chosen, several orbital optimisation algorithms are introduced. An energy-based algorithm using the simulated annealing method is used as a benchmark. As a computationally more affordable alternative, a seniority number minimising algorithm is developed and compared to the energy based one revealing that the seniority minimising orbital set performs well. Given a well-chosen orbital basis, it is shown that the newly developed DOCI based wave functions are especially suitable for the computationally efficient description of static correlation and to lesser extent dynamic correlation.Fil: Van Raemdonck, Mario. Ghent University; BélgicaFil: Alcoba, Diego Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Poelmans, Ward. Ghent University; BélgicaFil: De Baerdemacker, Stijn. Ghent University; BélgicaFil: Torre, Alicia. Universidad del País Vasco; EspañaFil: Lain, Luis. Universidad del País Vasco; EspañaFil: Massaccesi, Gustavo Ernesto. Universidad de Barcelona. Facultad de Física. Departamento de Física Fomental; EspañaFil: Van Neck, D.. Ghent University; BélgicaFil: Bultinck, P.. Ghent University; Bélgic
Few-body precursor of the Higgs mode in a superfluid Fermi gas
We demonstrate that an undamped few-body precursor of the Higgs mode can be
investigated in a harmonically trapped Fermi gas. Using exact diagonalisation,
the lowest monopole mode frequency is shown to depend non-monotonically on the
interaction strength, having a minimum in a crossover region. The minimum
deepens with increasing particle number, reflecting that the mode is the
few-body analogue of a many-body Higgs mode in the superfluid phase, which has
a vanishing frequency at the quantum phase transition point to the normal
phase. We show that this mode mainly consists of coherent excitations of
time-reversed pairs, and that it can be selectively excited by modulating the
interaction strength, using for instance a Feshbach resonance in cold atomic
gases.Comment: 9 pages, 7 figure
Symmetry-reinforced Nogood Recording from Restarts
dans le cadre de CP'11International audienceNogood recording from restarts is a form of lightweight learn- ing that combines nogood recording with a restart strategy. At the end of each run, nogoods are extracted from the current (rightmost) branch of the search tree. These nogoods can be used to prevent parts of the search space from being explored more than once. In this paper, we propose to reinforce nogood recording (from restarts) by exploiting symmetries: every time the solver has to be restarted, not only the nogoods that are extracted from the current branch are recorded, but also some additional nogoods that can be computed by means of the previously identi ed problem symmetries. This mechanism of computing symmetric nogoods can be iterated until a xed-point is reached, and controlled (if necessary) by limiting the number and/or the size of recorded nogoods
Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study
The effect of stress-triaxiality on growth of a void in a three dimensional
single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular
dynamics (MD) simulations using an embedded-atom (EAM) potential for copper
have been performed at room temperature and using strain controlling with high
strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these
magnitudes can be studied experimentally, e.g. using shock waves induced by
laser ablation. Void growth has been simulated in three different conditions,
namely uniaxial, biaxial, and triaxial expansion. The response of the system in
the three cases have been compared in terms of the void growth rate, the
detailed void shape evolution, and the stress-strain behavior including the
development of plastic strain. Also macroscopic observables as plastic work and
porosity have been computed from the atomistic level. The stress thresholds for
void growth are found to be comparable with spall strength values determined by
dynamic fracture experiments. The conventional macroscopic assumption that the
mean plastic strain results from the growth of the void is validated. The
evolution of the system in the uniaxial case is found to exhibit four different
regimes: elastic expansion; plastic yielding, when the mean stress is nearly
constant, but the stress-triaxiality increases rapidly together with
exponential growth of the void; saturation of the stress-triaxiality; and
finally the failure.Comment: 35 figures, which are small (and blurry) due to the space
limitations; submitted (with original figures) to Physical Review B. Final
versio
LATTICE DISTORTION NEAR VACANCIES IN DIAMOND AND SILICON .1.
A dynamical relaxation procedure, coupled with a valence force potential, has been used to calculate the distortion around point defects in a diamond-type crystal. The method has been applied to the vacancy in diamond and silicon. The response of the lattice to symmetrized forces on the nearest neighbours to the vacancy was calculated. The results can be used in estimates of point defect properties which depend on lattice distortion, including the jahn-teller effect, and formation energies. The ratios of the atomic displacements under uniform external stresses for the perfect lattice and for the lattice with a vacancy are also determined
The Non-Perturbative Quantum Nature of the Dislocation-Phonon Interaction
Despite the long history of dislocation-phonon interaction studies, there are
many problems that have not been fully resolved during this development. These
include an incompatibility between a perturbative approach and the long-range
nature of a dislocation, the relation between static and dynamic scattering,
and the nature of dislocation-phonon resonance. Here by introducing a fully
quantized dislocation field, the "dislon"[1], a phonon is renormalized as a
quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite
quasi-phonon lifetime that is reducible to classical results. A series of
outstanding legacy issues including those above can be directly explained
within this unified phonon renormalization approach. In particular, a
renormalized phonon naturally resolves the decades-long debate between dynamic
and static dislocation-phonon scattering approaches.Comment: 5 pages main text, 3 figures, 10 pages supplemental material
1D Effectively Closed Subshifts and 2D Tilings
Michael Hochman showed that every 1D effectively closed subshift can be
simulated by a 3D subshift of finite type and asked whether the same can be
done in 2D. It turned out that the answer is positive and necessary tools were
already developed in tilings theory. We discuss two alternative approaches:
first, developed by N. Aubrun and M. Sablik, goes back to Leonid Levin; the
second one, developed by the authors, goes back to Peter Gacs.Comment: Journ\'ees Automates Cellulaires, Turku : Finland (2010
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