1,369 research outputs found
Complex coupled-cluster approach to an ab-initio description of open quantum systems
We develop ab-initio coupled-cluster theory to describe resonant and weakly
bound states along the neutron drip line. We compute the ground states of the
helium chain 3-10He within coupled-cluster theory in singles and doubles (CCSD)
approximation. We employ a spherical Gamow-Hartree-Fock basis generated from
the low-momentum N3LO nucleon-nucleon interaction. This basis treats bound,
resonant, and continuum states on equal footing, and is therefore optimal for
the description of properties of drip line nuclei where continuum features play
an essential role. Within this formalism, we present an ab-initio calculation
of energies and decay widths of unstable nuclei starting from realistic
interactions.Comment: 4 pages, revtex
Medium-mass nuclei from chiral nucleon-nucleon interactions
We compute the binding energies, radii, and densities for selected
medium-mass nuclei within coupled-cluster theory and employ the "bare" chiral
nucleon-nucleon interaction at order N3LO. We find rather well-converged
results in model spaces consisting of 15 oscillator shells, and the doubly
magic nuclei 40Ca, 48Ca, and the exotic 48Ni are underbound by about 1 MeV per
nucleon within the CCSD approximation. The binding-energy difference between
the mirror nuclei 48Ca and 48Ni is close to theoretical mass table evaluations.
Our computation of the one-body density matrices and the corresponding natural
orbitals and occupation numbers provides a first step to a microscopic
foundation of the nuclear shell model.Comment: 5 pages, 5 figure
Computation of spectroscopic factors with the coupled-cluster method
We present a calculation of spectroscopic factors within coupled-cluster
theory. Our derivation of algebraic equations for the one-body overlap
functions are based on coupled-cluster equation-of-motion solutions for the
ground and excited states of the doubly magic nucleus with mass number and
the odd-mass neighbor with mass . As a proof-of-principle calculation, we
consider O and the odd neighbors O and N, and compute the
spectroscopic factor for nucleon removal from O. We employ a
renormalized low-momentum interaction of the type derived
from a chiral interaction at next-to-next-to-next-to-leading order. We study
the sensitivity of our results by variation of the momentum cutoff, and then
discuss the treatment of the center of mass.Comment: 8 pages, 6 figures, 3 table
Comment on "Ab Initio study of 40-Ca with an importance-truncated no-core shell model"
In a recent Letter [Phys. Rev. Lett. 99, 092501 (2007)], Roth and Navratil
present an importance-truncation scheme for the no-core shell model. The
authors claim that their truncation scheme leads to converged results for the
ground state of 40-Ca. We believe that this conclusion cannot be drawn from the
results presented in the Letter. Furthermore, the claimed convergence is at
variance with expectations of many-body theory. In particular, coupled-cluster
calculations indicate that a significant fraction of the correlation energy is
missing.Comment: 1 page, comment on arXiv:0705.4069 (PRL 99, 092501 (2007)
Telepresence and Space Station Freedom workstation operations
The Space Station Freedom workstation system is a distributed network of computer based workstations that provides the man-machine interfaces for controlling space station systems. This includes control of external manipulator, robotic and free flyer devices by crewmembers in the space station's pressurized shirt-sleeve environment. These remotely controlled devices help minimize the requirement for costly crew extravehicular activity (EVA) time for such tasks as station assembly and payload support. Direct window views may be used for controlling some of the systems, but many activities will be remote or require levels of detail not possible by direct observation. Since controlling remote devices becomes more difficult when direct views are inadequate or unavailable, many performance enhancing techniques have been considered for representing information about remote activities to the operator. Described here are the telepresence techniques under consideration to support operations and training. This includes video enhancements (e.g., graphic and text overlays and stereo viewing), machine vision systems, remote activity animation, and force reflection representation
Ab-initio computation of neutron-rich oxygen isotopes
We compute the binding energy of neutron-rich oxygen isotopes and employ the
coupled-cluster method and chiral nucleon-nucleon interactions at
next-to-next-to-next-to-leading order with two different cutoffs. We obtain
rather well-converged results in model spaces consisting of up to 21 oscillator
shells. For interactions with a momentum cutoff of 500 MeV, we find that 28O is
stable with respect to 24O, while calculations with a momentum cutoff of 600
MeV result in a slightly unbound 28O. The theoretical error estimates due to
the omission of the three-nucleon forces and the truncation of excitations
beyond three-particle-three-hole clusters indicate that the stability of 28O
cannot be ruled out from ab-initio calculations, and that three-nucleon forces
and continuum effects play the dominant role in deciding this question.Comment: 5 pages + eps, 3 figure
Nuclear Structure Calculations with Coupled Cluster Methods from Quantum Chemistry
We present several coupled-cluster calculations of ground and excited states
of 4He and 16O employing methods from quantum chemistry. A comparison of
coupled cluster results with the results of exact diagonalization of the
hamiltonian in the same model space and other truncated shell-model
calculations shows that the quantum chemistry inspired coupled cluster
approximations provide an excellent description of ground and excited states of
nuclei, with much less computational effort than traditional large-scale
shell-model approaches. Unless truncations are made, for nuclei like 16O,
full-fledged shell-model calculations with four or more major shells are not
possible. However, these and even larger systems can be studied with the
coupled cluster methods due to the polynomial rather than factorial scaling
inherent in standard shell-model studies. This makes the coupled cluster
approaches, developed in quantum chemistry, viable methods for describing
weakly bound systems of interest for future nuclear facilities.Comment: 10 pages, Elsevier latex style, Invited contribution to INPC04
proceedings, to appear in Nuclear Physics
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