108,245 research outputs found
Torsional anharmonicity in the conformational thermodynamics of flexible molecules
We present an algorithm for calculating the conformational thermodynamics of large, flexible molecules that combines ab initio electronic structure theory calculations with a torsional path integral Monte Carlo (TPIMC) simulation. The new algorithm overcomes the previous limitations of the TPIMC method by including the thermodynamic contributions of non-torsional vibrational modes and by affordably incorporating the ab initio calculation of conformer electronic energies, and it improves the conventional ab initio treatment of conformational thermodynamics by accounting for the anharmonicity of the torsional modes. Using previously published ab initio results and new TPIMC calculations, we apply the algorithm to the conformers of the adrenaline molecule
A hierarchical research by large-scale and ab initio electronic structure theories -- Si and Ge cleavage and stepped (111)-2x1 surfaces --
The ab initio calculation with the density functional theory and plane-wave
bases is carried out for stepped Si(111)-2x1 surfaces that were predicted in a
cleavage simulation by the large-scale (order-N) electronic structure theory
(T. Hoshi, Y. Iguchi and T. Fujiwara, Phys. Rev. B72 (2005) 075323). The
present ab initio calculation confirms the predicted stepped structure and its
bias-dependent STM image. Moreover, two (meta)stable step-edge structures are
found and compared. The investigation is carried out also for Ge(111)-2x1
surfaces, so as to construct a common understanding among elements. The present
study demonstrates the general importance of the hierarchical research between
large-scale and ab initio electronic structure theories.Comment: 5 pages, 4 figures, to appear in Physica
Thermodynamic stability of Fe/O solid solution at inner-core conditions
We present a new technique which allows the fully {\em ab initio} calculation
of the chemical potential of a substitutional impurity in a high-temperature
crystal, including harmonic and anharmonic lattice vibrations. The technique
uses the combination of thermodynamic integration and reference models
developed recently for the {\em ab initio} calculation of the free energy of
liquids and anharmonic solids. We apply the technique to the case of the
substitutional oxygen impurity in h.c.p. iron under Earth's core conditions,
which earlier static {\em ab initio} calculations indicated to be
thermodynamically very unstable. Our results show that entropic effects arising
from the large vibrational amplitude of the oxygen impurity give a major
reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p.
iron may be stabilised at concentrations up a few mol % under core conditions
Ab initio calculation of Li7 photodisintegration
The Li7 total photoabsorption cross section is calculated microscopically. As
nucleon-nucleon interaction the semi-realistic central AV4' potential with S-
and P-wave forces is taken. The interaction of the final 7-nucleon system is
fully taken into account via the Lorentz Integral Transform (LIT) method. For
the calculation of the LIT we use expansions in hyperspherical harmonics (HH)
in conjunction with the HH effective interaction (EIHH) approach. The
convergence of the LIT expansion is discussed in detail. The calculated cross
section agrees quite well with the available experimental data, which cover an
energy range from threshold up to 100 MeV.Comment: 11 pages with 3 figure
The vibrational dynamics of vitreous silica: Classical force fields vs. first-principles
We compare the vibrational properties of model SiO_2 glasses generated by
molecular-dynamics simulations using the effective force field of van Beest et
al. (BKS) with those obtained when the BKS structure is relaxed using an ab
initio calculation in the framework of the density functional theory. We find
that this relaxation significantly improves the agreement of the density of
states with the experimental result. For frequencies between 14 and 26 THz the
nature of the vibrational modes as determined from the BKS model is very
different from the one from the ab initio calculation, showing that the
interpretation of the vibrational spectra in terms of calculations using
effective potentials can be very misleading.Comment: 7 pages of Latex, 4 figure
Ab initio calculation of the Hoyle state
The Hoyle state plays a crucial role in the hydrogen burning of stars heavier
than our sun and in the production of carbon and other elements necessary for
life. This excited state of the carbon-12 nucleus was postulated by Hoyle [1]
as a necessary ingredient for the fusion of three alpha particles to produce
carbon at stellar temperatures. Although the Hoyle state was seen
experimentally more than a half century ago [2,3], nuclear theorists have not
yet uncovered the nature of this state from first principles. In this letter we
report the first ab initio calculation of the low-lying states of carbon-12
using supercomputer lattice simulations and a theoretical framework known as
effective field theory. In addition to the ground state and excited spin-2
state, we find a resonance at -85(3) MeV with all of the properties of the
Hoyle state and in agreement with the experimentally observed energy. These
lattice simulations provide insight into the structure of this unique state and
new clues as to the amount of fine-tuning needed in nature for the production
of carbon in stars.Comment: 4 pp, 3 eps figs, version accepted for publication in Physical Review
Letter
Quantum Monte Carlo study of inhomogeneous neutron matter
We present an ab-initio study of neutron drops. We use Quantum Monte Carlo
techniques to calculate the energy up to 54 neutrons in different external
potentials, and we compare the results with Skyrme forces. We also calculate
the rms radii and radial densities, and we find that a re-adjustment of the
gradient term in Skyrme is needed in order to reproduce the properties of these
systems given by the ab-initio calculation. By using the ab-initio results for
neutron drops for close- and open-shell configurations, we suggest how to
improve Skyrme forces when dealing with systems with large isospin-asymmetries
like neutron-rich nuclei.Comment: 8 pages, 6 figures, talk given at Horizons on Innovative Theories,
Experiments, and Supercomputing in Nuclear Physics 2012, (HITES2012), New
Orleans, Louisiana, June 4-7, 2012; to appear in Journal of Physics:
Conference Series (JPCS
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