1,950 research outputs found
The spectra of mixed He-He droplets
The diffusion Monte Carlo technique is used to calculate and analyze the
excitation spectrum of He atoms bound to a cluster of He atoms, by
using a previously determined optimum filling of single-fermion orbits with
well defined orbital angular momentum , spin and parity quantum numbers.
The study concentrates on the energies and shapes of the three kinds of states
for which the fermionic part of the wave function is a single Slater
determinant: maximum or maximum states within a given orbit, and fully
polarized clusters. The picture that emerges is that of systems with strong
shell effects whose binding and excitation energies are essentially determined
over configuration at fixed number of particles and spin, i.e., by the monopole
properties of an effective Hamiltonian.Comment: 14 pages, 15 figure
Nilsson-SU3 selfconsistency in heavy N=Z nuclei
It is argued that there exist natural shell model spaces optimally adapted to
the operation of two variants of Elliott' SU3 symmetry that provide accurate
predictions of quadrupole moments of deformed states. A selfconsistent
Nilsson-like calculation describes the competition between the realistic
quadrupole force and the central field, indicating a {\em remarkable stability
of the quadruplole moments}---which remain close to their quasi and pseudo SU3
values---as the single particle splittings increase. A detailed study of the
even nuclei from Ni to Cd reveals that the region of
prolate deformation is bounded by a pair of transitional nuclei Kr and
Mo in which prolate ground state bands are predicted to dominate, though
coexisting with oblate ones,Comment: Replacement I) Title simplified. II) Major revision: structure of
paper kept but two thirds totally rewritten (same number of pages); 20
references adde
Coulomb displacement energies, energy differenced and neutron skins
A Fock space representation of the monopole part of the Coulomb potential is
presented. Quantum effects show through a small orbital term in . Once
it is averaged out, the classical electrostatic energy emerges as an
essentially exact expression, which makes it possible to eliminate the
Nolen-Schiffer anomaly, and to estimate neutron skins and the evolution of
radii along yrast states of mirror nuclei. The energy differences of the latter
are quantitatively reproduced by the monopole term and a schematic multipole
one.Comment: 4 pages, 3 figures, Revte
Three-body monopole corrections to the realistic interactions
It is shown that a very simple three-body monopole term can solve practically
all the spectroscopic problems--in the , and shells--that were
hitherto assumed to need drastic revisions of the realistic potentials.Comment: 4 pages, 5figure
Mirror displacement energies and neutron skins
A gross estimate of the neutron skin [0.80(5) fm] is extracted from
experimental proton radii, represented by a four parameter fit, and observed
mirror displacement energies (CDE). The calculation of the latter relies on an
accurately derived Coulomb energy and smooth averages of the charge symmetry
breaking potentials constrained to state of the art values. The only free
parameter is the neutron skin itself. The Nolen Schiffer anomaly is reduced to
small deviations (rms=127 keV) that exhibit a secular trend. It is argued that
with state of the art shell model calculations the anomaly should disappear.
Highly accurate fits to proton radii emerge as a fringe benefit.Comment: 4 pages 3 figures, superseeds first part of nucl-th/0104048 Present
is new extended version: 5 pages 4 figures. Explains more clearly the
achievements of the previous on
Binomial level densities
It is shown that nuclear level densities in a finite space are described by a
continuous binomial function, determined by the first three moments of the
Hamiltonian, and the dimensionality of the underlying vector space.
Experimental values for Mn, Fe, and Ni are very well
reproduced by the binomial form, which turns out to be almost perfectly
approximated by Bethe's formula with backshift. A proof is given that binomial
densities reproduce the low moments of Hamiltonians of any rank: A strong form
of the famous central limit result of Mon and French. Conditions under which
the proof may be extended to the full spectrum are examined.Comment: 4 pages 2 figures Second version (previous not totally superseeded
Quantification of the differences between quenched and annealed averaging for RNA secondary structures
The analytical study of disordered system is usually difficult due to the
necessity to perform a quenched average over the disorder. Thus, one may resort
to the easier annealed ensemble as an approximation to the quenched system. In
the study of RNA secondary structures, we explicitly quantify the deviation of
this approximation from the quenched ensemble by looking at the correlations
between neighboring bases. This quantified deviation then allows us to propose
a constrained annealed ensemble which predicts physical quantities much closer
to the results of the quenched ensemble without becoming technically
intractable.Comment: 9 pages, 14 figures, submitted to Phys. Rev.
Analytical description of finite size effects for RNA secondary structures
The ensemble of RNA secondary structures of uniform sequences is studied
analytically. We calculate the partition function for very long sequences and
discuss how the cross-over length, beyond which asymptotic scaling laws apply,
depends on thermodynamic parameters. For realistic choices of parameters this
length can be much longer than natural RNA molecules. This has to be taken into
account when applying asymptotic theory to interpret experiments or numerical
results.Comment: 10 pages, 13 figures, published in Phys. Rev.
An Improved Algorithm for RNA Secondary Structure Prediction
Though not as abundant in known biological processes as proteins,RNA molecules serve as more than mere intermediaries betweenDNA and proteins, e.g. as catalytic molecules. Furthermore,RNA secondary structure prediction based on free energyrules for stacking and loop formation remains one of the few majorbreakthroughs in the field of structure prediction. We present anew method to evaluate all possible internal loops of size at mostk in an RNA sequence, s, in time O(k|s|^2); this is an improvementfrom the previously used method that uses time O(k^2|s|^2).For unlimited loop size this improves the overall complexity ofevaluating RNA secondary structures from O(|s|^4) to O(|s|^3) andthe method applies equally well to finding the optimal structureand calculating the equilibrium partition function. We use ourmethod to examine the soundness of setting k = 30, a commonlyused heuristic
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