100 research outputs found
Converging High-Level Coupled-Cluster Energetics via Adaptive Selection of Excitation Manifolds Driven by Moment Expansions
A novel approach to rapidly converging high-level coupled-cluster (CC)
energetics in an automated fashion is proposed. The key idea is an adaptive
selection of the excitation manifolds defining higher-than-two-body components
of the cluster operator inspired by the CC(;) moment expansions. The
usefulness of the resulting methodology is illustrated by molecular examples
where the goal is to recover the electronic energies obtained using the CC
method with a full treatment of singly, doubly, and triply excited clusters
(CCSDT) when the noniterative triples corrections to CCSD fail.Comment: 18 pages, 5 tables. This article has been accepted for publication in
the Journal of Chemical Physics. After it is published, it will be found at
https://doi.org/10.1063/5.016287
Interaction matrix element fluctuations in quantum dots
In the Coulomb blockade regime of a ballistic quantum dot, the distribution
of conductance peak spacings is well known to be incorrectly predicted by a
single-particle picture; instead, matrix element fluctuations of the residual
electronic interaction need to be taken into account. In the normalized
random-wave model, valid in the semiclassical limit where the number of
electrons in the dot becomes large, we obtain analytic expressions for the
fluctuations of two-body and one-body matrix elements. However, these
fluctuations may be too small to explain low-temperature experimental data. We
have examined matrix element fluctuations in realistic chaotic geometries, and
shown that at energies of experimental interest these fluctuations generically
exceed by a factor of about 3-4 the predictions of the random wave model. Even
larger fluctuations occur in geometries with a mixed chaotic-regular phase
space. These results may allow for much better agreement between the
Hartree-Fock picture and experiment. Among other findings, we show that the
distribution of interaction matrix elements is strongly non-Gaussian in the
parameter range of experimental interest, even in the random wave model. We
also find that the enhanced fluctuations in realistic geometries cannot be
computed using a leading-order semiclassical approach, but may be understood in
terms of short-time dynamics.Comment: 12 pages, 6 figures; submitted for conference proceedings of Workshop
on Nuclei and Mesoscopic Physics (WNMP07), October 20-22, 2007, East Lansing,
Michigan (Pawel Danielewicz, Editor
How changing physical constants and violation of local position invariance may occur?
Light scalar fields very naturally appear in modern cosmological models,
affecting such parameters of Standard Model as electromagnetic fine structure
constant , dimensionless ratios of electron or quark mass to the QCD
scale, . Cosmological variations of these scalar fields
should occur because of drastic changes of matter composition in Universe: the
latest such event is rather recent (redshift ), from matter to dark
energy domination. In a two-brane model (we use as a pedagogical example) these
modifications are due to changing distance to "the second brane", a massive
companion of "our brane". Back from extra dimensions, massive bodies (stars or
galaxies) can also affect physical constants. They have large scalar charge
proportional to number of particles which produces a Coulomb-like scalar
field . This leads to a variation of the fundamental constants
proportional to the gravitational potential, e.g. . We compare different manifestations of this
effect. The strongest limits are
obtained from the measurements of dependence of atomic frequencies on the
distance from Sun (the distance varies due to the ellipticity of the Earth's
orbit).Comment: reference adde
Extension of coupled-cluster theory with a non-iterative treatment of connected triply excited clusters to three-body Hamiltonians
We generalize the coupled-cluster (CC) approach with singles, doubles, and
the non-iterative treatment of triples termed CCSD(T) to Hamiltonians
containing three-body interactions. The resulting method and the underlying CC
approach with singles and doubles only (CCSD) are applied to the medium-mass
closed-shell nuclei O16, O24, and Ca40. By comparing the results of CCSD and
CCSD(T) calculations with explicit treatment of three-nucleon
interactions to those obtained using an approximate treatment in which they are
included effectively via the zero-, one-, and two-body components of the
Hamiltonian in normal-ordered form, we quantify the contributions of the
residual three-body interactions neglected in the approximate treatment. We
find these residual normal-ordered three-body contributions negligible for the
CCSD(T) method, although they can become significant in the
lower-level CCSD approach, particularly when the nucleon-nucleon interactions
are soft.Comment: 21 pages, 3 figure
Mesoscopic Fluctuations of the Pairing Gap
A description of mesoscopic fluctuations of the pairing gap in finite-sized
quantum systems based on periodic orbit theory is presented. The size of the
fluctuations are found to depend on quite general properties. We distinguish
between systems where corresponding classical motion is regular or chaotic, and
describe in detail fluctuations of the BCS gap as a function of the size of the
system. The theory is applied to different mesoscopic systems: atomic nuclei,
metallic grains, and ultracold fermionic gases. We also present a detailed
description of pairing gap variation with particle number for nuclei based on a
deformed cavity potential.Comment: Conference Proceeding of Mesoscopic Workshop WNMP0
Center-of-mass problem in truncated configuration interaction and coupled-cluster calculations
The problem of center-of-mass (CM) contaminations in ab initio nuclear
structure calculations using configuration interaction (CI) and coupled-cluster
(CC) approaches is analyzed. A rigorous and quantitative scheme for diagnosing
the CM contamination of intrinsic observables is proposed and applied to
ground-state calculations for He-4 and O-16. The CI and CC calculations for
O-16 based on model spaces defined via a truncation of the single-particle
basis lead to sizable CM contaminations, while the importance-truncated no-core
shell model based on the space is virtually free of CM
contaminations.Comment: 6 pages, 1 figure, 2 tables; v2: minor modifications, Phys. Lett. B
in prin
Decay Rate Statistics of Unstable Classically Chaotic Systems
Decay law of a complicated unstable state formed in a high energy collision
is described by the Fourier transform of the two-point correlation function of
the scattering matrix. Although each constituent resonance state decays
exponentially the decay of a state composed of a large number of such
interfering resonances is not, generally, exponential. We introduce the decay
rates distribution function by representing the decay law in the form of the
mean-weighted decay exponent. In the framework of the random matrix approach we
investigate the properties of the new distribution function and its relation to
the more conventional statistics of the decay widths. The latter is not in fact
conclusive as concerns the evolution during the time shorter than the
characteristic Heisenberg time. Exact analytical consideration is presented for
the case of systems without time reversal symmetry.Comment: 12 page
Breaking Bonds with the Left Eigenstate Completely Renormalized Coupled-Cluster Method
The recently developed [P. Piecuch and M. Wloch, J. Chem. Phys.123, 224105 (2005)] size-extensive left eigenstate completely renormalized (CR) coupled-cluster (CC) singles (S), doubles (D), and noniterative triples (T) approach, termed CR-CC(2,3) and abbreviated in this paper as CCL, is compared with the full configuration interaction (FCI) method for all possible types of single bond-breaking reactions between C, H, Si, and Cl (except H2) and the H2SiSiH2 double bond-breaking reaction. The CCL method is in excellent agreement with FCI in the entire region R=1–3Re for all of the studied single bond-breaking reactions, whereR and Re are the bond distance and the equilibrium bond length, respectively. The CCL method recovers the FCI results to within approximately 1mhartree in the region R=1–3Reof the H–SiH3, H–Cl, H3Si–SiH3, Cl–CH3, H–CH3, and H3C–SiH3bonds. The maximum errors are −2.1, 1.6, and 1.6mhartree in the R=1–3Re region of the H3C–CH3, Cl–Cl, and H3Si–Clbonds, respectively, while the discrepancy for the H2SiSiH2 double bond-breaking reaction is 6.6 (8.5)mhartree at R=2(3)Re. CCL also predicts more accurate relative energies than the conventional CCSD and CCSD(T) approaches, and the predecessor of CR-CC(2,3) termed CR-CCSD(T)
Symmetry Breaking Study with Random Matrix Ensembles
A random matrix model to describe the coupling of -fold symmetry is
constructed. The particular threefold case is used to analyze data on
eigenfrequencies of elastomechanical vibration of an anisotropic quartz block.
It is suggested that such experimental/theoretical study may supply a powerful
means to discern intrinsic symmetry of physical systems.Comment: 12 pages, 3 figures Contribution to the International Workshop on
Nuclei and Mesoscopic Physics (WNM07), 20-22 October, Michigan Sate
University, East Lansing, Michigan. To appear in a AIP Proceeding (Pawel
Danielewicz, Editor
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