110,990 research outputs found
Torsional path integral Monte Carlo method for calculating the absolute quantum free energy of large molecules
A new technique for evaluating the absolute free energy of large molecules is presented. Quantum-mechanical contributions to the intramolecular torsions are included via the torsional path integral Monte Carlo (TPIMC) technique. Importance sampling schemes based on uncoupled free rotors and harmonic oscillators facilitate the use of the TPIMC technique for the direct evaluation of quantum partition functions. Absolute free energies are calculated for the molecules ethane, n-butane, n-octane, and enkephalin, and quantum contributions are found to be significant. Comparison of the TPIMC technique with the harmonic oscillator approximation and a variational technique is performed for the ethane molecule. For all molecules, the quantum contributions to free energy are found to be significant but slightly smaller than the quantum contributions to internal energy
Quantum free energies of the conformers of glycine on an ab initio potential energy surface
The torsional path integral Monte Carlo (TPIMC) technique is used to study the five lowest-energy conformers of glycine. The theoretical method provides an anharmonic and quantum-mechanical description of conformational free energy and is used for the first time with an ab initio potential energy surface. The 3-dimensional torsional potential energy surface of glycine was obtained at the MP2/6-311++G** level of theory and is optimized with respect to the non-torsional degrees of freedom. Calculated conformer populations compare well with those reported in recent matrix-isolation infrared spectroscopy experiments. An additional conformer, not yet observed, is predicted to be heavily populated in the thermal equilibria probed by experiment, and a new explanation for its elusiveness is provided. Quantum effects, such as zero point energy, are found to substantially alter conformer populations, and an algorithm for estimating the role of non-torsional vibrations in the conformational thermodynamics of a molecule is introduced
Torsional path integral Monte Carlo method for the quantum simulation of large molecules
A molecular application is introduced for calculating quantum statistical mechanical expectation values of large molecules at nonzero temperatures. The Torsional Path Integral Monte Carlo (TPIMC) technique applies an uncoupled winding number formalism to the torsional degrees of freedom in molecular systems. The internal energy of the molecules ethane, n-butane, n-octane, and enkephalin are calculated at standard temperature using the TPIMC technique and compared to the expectation values obtained using the harmonic oscillator approximation and a variational technique. All studied molecules exhibited significant quantum mechanical contributions to their internal energy expectation values according to the TPIMC technique. The harmonic oscillator approximation approach to calculating the internal energy performs well for the molecules presented in this study but is limited by its neglect of both anharmonicity effects and the potential coupling of intramolecular torsion
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
Probing QCD approach to thermal equilibrium with ultrahigh energy cosmic rays
The Pierre Auger Collaboration has reported an excess in the number of muons
of a few tens of percent over expectations computed using extrapolation of
hadronic interaction models tuned to accommodate LHC data. Very recently, we
proposed an explanation for the muon excess assuming the formation of a
deconfined quark matter (fireball) state in central collisions of
ultrarelativistic cosmic rays with air nuclei. At the first stage of its
evolution the fireball contains gluons as well as and quarks. The very
high baryochemical potential inhibits gluons from fragmenting into
and , and so they fragment predominantly into pairs. In
the hadronization which follows this leads to the strong suppression of pions
and hence photons, but allows heavy hadrons to be emitted carrying away
strangeness. In this manner, the extreme imbalance of hadron to photon content
provides a way to enhance the muon content of the air shower. In this
communication we study theoretical systematics from hadronic interaction models
used to describe the cascades of secondary particles produced in the fireball
explosion. We study the predictions of one of the leading LHC-tuned models
QGSJET II-04 considered in the Auger analysis.Comment: 7 pages LaTeX, 6 .pdf figure
Meson-Nucleon Vertex Form Factors at Finite Temperature Using a Soft Pion Form Factor
The temperature and density dependence of the meson-nucleon vertex form
factors is studied in the framework of thermofield dynamics. Results are
obtained for two rather different nucleon-nucleon potentials: the usual Bonn
potential and the variation with a softer form factor, due to Holinde
and Thomas. In general, the results show only a modest degree of sensitivity to
the choice of interaction.Comment: 5 pages, uses revtex
Early Results on Radioactive Background Characterization for Sanford Laboratory and DUSEL Experiments
Measuring external sources of background for a deep underground laboratory at
the Homestake Mine is an important step for the planned low-background
experiments. The naturally occurring -ray fluxes at different levels in
the Homestake Mine are studied using NaI detectors and Monte Carlo simulations.
A simple algorithm is developed to convert the measured -ray rates into
-ray fluxes. A good agreement between the measured and simulated
-ray fluxes is achieved with the knowledge of the chemical composition
and radioactivity levels in the rock. The neutron fluxes and -ray
fluxes are predicted by Monte Carlo simulations for different levels including
inaccessible levels that are under construction for the planned low background
experiments.Comment: 16 pages, 2 figures, and 9 table
Dynamics of hadron strong production and decay
We generalize results of lattice QCD to determine the spin-dependent
symmetries and factorization properties of meson production in OZI allowed
processes. This explains some conjectures previously made in the literature
about axial meson decays and gives predictions for exclusive decays of vector
charmonia, including ways of establishing the structure of Y(4260) and Y(4325)
from their S-wave decays. Factorization gives a selection rule which forbids
near threshold with the tensor meson in helicity 2. The
relations among amplitudes for double charmonia production \e^+e^-\to
\psi\chi_{0,1,2} are expected to differ from the analagous relations among
light flavour production such as \e^+e^-\to \omega f_{0,1,2}.Comment: 13 pages; journal versio
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