201 research outputs found
The exclusive (e,ep) reaction at high missing momenta
The reduced (e,ep) cross section is calculated for kinematics that probe
high missing momenta. The final-state interaction is handled within a
non-relativistic many-body framework. One- and two-body nuclear currents are
included. Electron distortion effects are treated in an exact distorted wave
calculation. It is shown that at high missing momenta the calculated (e,ep)
cross sections exhibit a pronounced sensitivity to ground-state correlations of
the RPA type and two-body currents. The role of these mechanisms is found to be
relatively small at low missing momenta.Comment: 15 pages in REVtex with embedded psfigure
On the nuclear symmetry energy and the neutron skin in neutron-rich nuclei
The symmetry energy for nuclear matter and its relation to the neutron skin
in finite nuclei is discussed. The symmetry energy as a function of density
obtained in a self-consistent Green function approach is presented and compared
to the results of other recent theoretical approaches. A partial explanation of
the linear relation between the symmetry energy and the neutron skin is
proposed. The potential of several experimental methods to extract the neutron
skin is examined.Comment: to appear in Phys. Rev.
Center-of-mass effects on the quasi-hole spectroscopic factors in the 16O(e,e'p) reaction
The spectroscopic factors for the low-lying quasi-hole states observed in the
16O(e,e'p)15N reaction are reinvestigated with a variational Monte Carlo
calculation for the structure of the initial and final nucleus. A computational
error in a previous report is rectified. It is shown that a proper treatment of
center-of-mass motion does not lead to a reduction of the spectroscopic factor
for -shell quasi-hole states, but rather to a 7% enhancement. This is in
agreement with analytical results obtained in the harmonic oscillator model.
The center-of-mass effect worsens the discrepancy between present theoretical
models and the experimentally observed single-particle strength. We discuss the
present status of this problem, including some other mechanisms that may be
relevant in this respect.Comment: 14 pages, no figures, uses Revtex, to be published in Phys. Rev. C 58
(1998
Efficient construction of free energy profiles of breathing metal–organic frameworks using advanced molecular dynamics simulations
In order to reliably predict and understand the breathing behavior of highly flexible metal–organic frameworks from thermodynamic considerations, an accurate estimation of the free energy difference between their different metastable states is a prerequisite. Herein, a variety of free energy estimation methods are thoroughly tested for their ability to construct the free energy profile as a function of the unit cell volume of MIL-53(Al). The methods comprise free energy perturbation, thermodynamic integration, umbrella sampling, metadynamics, and variationally enhanced sampling. A series of molecular dynamics simulations have been performed in the frame of each of the five methods to describe structural transformations in flexible materials with the volume as the collective variable, which offers a unique opportunity to assess their computational efficiency. Subsequently, the most efficient method, umbrella sampling, is used to construct an accurate free energy profile at different temperatures for MIL-53(Al) from first principles at the PBE+D3(BJ) level of theory. This study yields insight into the importance of the different aspects such as entropy contributions and anharmonic contributions on the resulting free energy profile. As such, this thorough study provides unparalleled insight in the thermodynamics of the large structural deformations of flexible materials
Electromagnetic interaction in chiral quantum hadrodynamics and decay of vector and axial-vector mesons
The chiral invariant QHD-III model of Serot and Walecka is applied in the calculation of some meson properties. The electromagnetic interaction is included by extending the symmetry of the model to the local U(1) \times SU(2)_{R} \times SU(2)_{L} group. The minimal and nonminimal contributions to the electromagnetic Lagrangian are obtained in a new representation of QHD-III. Strong decays of the axial-vector meson, a_{1} \to \pi \rho, a_{1} \to \pi \sigma, and the electromagnetic decays \rho \to \pi \pi \gamma, a_{1} \to \pi \gamma and \rho \to \pi \gamma are calculated. The low-energy parameters for the \pi-\pi scattering are calculated in the tree-level approximation. The effect of the auxiliary Higgs bosons, introduced in QHD-III in order to generate masses of the vector and axial-vector mesons via the Higgs mechanism, is studied as well. This is done on the tree level for \pi-\pi scattering and on the level of one-loop diagrams for the a_{1} \to \pi \gamma decay. It is demonstrated that the model successfully describes some features of meson phenomenology in the non-strange sector
A theoretical and experimental spectroscopy study on methanol and ethanol conversion over H-SAPO-34
The elucidation of the structure-activity relation of zeolites or zeotype materials remains very challenging. Recent advances in both theoretical and experimental techniques provide new opportunities to study these complex materials and any catalytic reaction occurring inside. In order to establish new active reaction routes, the knowledge of formed intermediates is crucial. The characterization of such intermediates can be done using a variety of spectroscopic techniques. In this contribution, methanol and ethanol conversion over H-SAPO-34 is investigated using IR and UV-VIS measurements. Calculated adsorption enthalpies of methanol and ethanol in a large SAPO 44T finite cluster show the stronger adsorption of the larger alcohol by 14 kJ mol-1. Dispersion contributions are found to be crucial. IR spectra are calculated for the clusters containing the adsorbed alcohols and matched with experimental data. In addition, the cluster is also loaded with singly methylated cationic hydrocarbons as these are representative reaction intermediates. A detailed normal mode analysis is performed, enabling to separate the framework-guest contributions. Based on the computed data in situ DRIFT experimental peaks could be assigned. Finally, contemporary DFT functionals such as CAM-B3LYP seem promising to compute gas phase UV-VIS spectra
Long-range correlations in finite nuclei: comparison of two self-consistent treatments
Long-range correlations, which are partially responsible for the observed fragmentation and depletion of low-lying single-particle strength, are studied in the Green's function formalism. The self-energy is expanded up to second order in the residual interaction. We compare two methods of implementing self-consistency in the solution of the Dyson equation beyond Hartree-Fock, for the case of the 16O nucleus. It is found that the energy-bin method and the BAGEL method lead to globally equivalent results. In both methods the final single-particle strength functions are characterized by exponential tails at energies far from the Fermi level
Minimal Basis Iterative Stockholder: Atoms in Molecules for Force-Field Development
Atomic partial charges appear in the Coulomb term of many force-field models
and can be derived from electronic structure calculations with a myriad of
atoms-in-molecules (AIM) methods. More advanced models have also been proposed,
using the distributed nature of the electron cloud and atomic multipoles. In
this work, an electrostatic force field is defined through a concise
approximation of the electron density, for which the Coulomb interaction is
trivially evaluated. This approximate "pro-density" is expanded in a minimal
basis of atom-centered s-type Slater density functions, whose parameters are
optimized by minimizing the Kullback-Leibler divergence of the pro-density from
a reference electron density, e.g. obtained from an electronic structure
calculation. The proposed method, Minimal Basis Iterative Stockholder (MBIS),
is a variant of the Hirshfeld AIM method but it can also be used as a
density-fitting technique. An iterative algorithm to refine the pro-density is
easily implemented with a linear-scaling computational cost, enabling
applications to supramolecular systems. The benefits of the MBIS method are
demonstrated with systematic applications to molecular databases and extended
models of condensed phases. A comparison to 14 other AIM methods shows its
effectiveness when modeling electrostatic interactions. MBIS is also suitable
for rescaling atomic polarizabilities in the Tkatchenko-Sheffler scheme for
dispersion interactions.Comment: 61 pages, 12 figures, 2 table
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