13,894 research outputs found
Ab initio investigation of intermolecular interactions in solid benzene
A computational strategy for the evaluation of the crystal lattice constants
and cohesive energy of the weakly bound molecular solids is proposed. The
strategy is based on the high level ab initio coupled-cluster determination of
the pairwise additive contribution to the interaction energy. The
zero-point-energy correction and non-additive contributions to the interaction
energy are treated using density functional methods. The experimental crystal
lattice constants of the solid benzene are reproduced, and the value of 480
meV/molecule is calculated for its cohesive energy
Shearing Box Simulations of the MRI in a Collisionless Plasma
We describe local shearing box simulations of turbulence driven by the
magnetorotational instability (MRI) in a collisionless plasma. Collisionless
effects may be important in radiatively inefficient accretion flows, such as
near the black hole in the Galactic Center. The MHD version of ZEUS is modified
to evolve an anisotropic pressure tensor. A fluid closure approximation is used
to calculate heat conduction along magnetic field lines. The anisotropic
pressure tensor provides a qualitatively new mechanism for transporting angular
momentum in accretion flows (in addition to the Maxwell and Reynolds stresses).
We estimate limits on the pressure anisotropy due to pitch angle scattering by
kinetic instabilities. Such instabilities provide an effective ``collision''
rate in a collisionless plasma and lead to more MHD-like dynamics. We find that
the MRI leads to efficient growth of the magnetic field in a collisionless
plasma, with saturation amplitudes comparable to those in MHD. In the saturated
state, the anisotropic stress is comparable to the Maxwell stress, implying
that the rate of angular momentum transport may be moderately enhanced in a
collisionless plasma.Comment: 20 pages, 9 figures, submitted to Ap
Recent developments of MCViNE and its applications at SNS
MCViNE is an open source, object-oriented Monte Carlo neutron ray-tracing simulation software package. Its design allows for flexible, hierarchical representations of sophisticated instrument components such as detector systems, and samples with a variety of shapes and scattering kernels. Recently this flexible design has enabled several applications of MCViNE simulations at the Spallation Neutron Source (SNS) at Oak Ridge National Lab, including assisting design of neutron instruments at the second target station and design of novel sample environments, as well as studying effects of instrument resolution and multiple scattering. Here we provide an overview of the recent developments and new features of MCViNE since its initial introduction (Jiao et al 2016 Nucl. Instrum. Methods Phys. Res., Sect. A 810, 86–99), and some example applications
Confirmation of Parity Violation in the Gamma Decay of
This paper reports measurements using the technique of On Line Nuclear
Orientation (OLNO) which reexamine the gamma decay of isomeric Hf and specifically the 501 keV 8 -- 6
transition. The irregular admixture of E2 to M2/E3 multipolarity in this
transition, deduced from the forward-backward asymmetry of its angular
distribution, has for decades stood as the prime evidence for parity mixing in
nuclear states. The experiment, based on ion implantation of the newly
developed mass-separated Hf beam at ISOLDE, CERN into an
iron foil maintained at millikelvin temperatures, produces higher degrees of
polarization than were achieved in previous studies of this system. The value
found for the E2/M2 mixing ratio, = -0.0324(16)(17), is in close
agreement with the previous published average value = - 0.030(2), in
full confirmation of the presence of the irregular E2 admixture in the 501 keV
transition. The temperature dependence of the forward-backward asymmetry has
been measured over a more extended range of nuclear polarization than
previously possible, giving further evidence for parity mixing of the 8 and 8 levels and the deduced E2/M2 mixing ratio.Comment: 28 pages, 9 figures, accepted for publication in Physical Review
Chemistry and radiative shielding in star forming galactic disks
To understand the conditions under which dense, molecular gas is able to form
within a galaxy, we post-process a series of three-dimensional
galactic-disk-scale simulations with ray-tracing based radiative transfer and
chemical network integration to compute the equilibrium chemical and thermal
state of the gas. In performing these simulations we vary a number of
parameters, such as the ISRF strength, vertical scale height of stellar
sources, cosmic ray flux, to gauge the sensitivity of our results to these
variations. Self-shielding permits significant molecular hydrogen (H2)
abundances in dense filaments around the disk midplane, accounting for
approximately ~10-15% of the total gas mass. Significant CO fractions only form
in the densest, n>~10^3 cm^-3, gas where a combination of dust, H2, and
self-shielding attenuate the FUV background. We additionally compare these
ray-tracing based solutions to photochemistry with complementary models where
photo-shielding is accounted for with locally computed prescriptions. With some
exceptions, these local models for the radiative shielding length perform
reasonably well at reproducing the distribution and amount of molecular gas as
compared with a detailed, global ray tracing calculation. Specifically, an
approach based on the Jeans Length with a T=40K temperature cap performs the
best in regards to a number of different quantitative measures based on the H2
and CO abundances.Comment: 21 Pages, 15 figures. Submitted to MNRAS. Comments welcom
Eficiência de uso de água pela cultura do milho (Zea mays) em função da cobertura do solo pela palhada no sistema plantio direto.
A maior eficiência do uso de água proporcionada pelo sistema plantio direto é devida à presença de uma adequada proteção da superfÃcie do solo, ao reduzir as perdas por evaporação. O objetivo desse trabalho foi avaliar diferentes métodos de manejo da irrigação em solos cobertos pela palhada na eficiência de uso de água pela cultura do milho verde (Zea mays L.), cultivada no SPD. Os tratamentos foram a combinação de três métodos de manejo da irrigação (1-Tensiômetro e curva de retenção de água, 2-Tanque Classe ?A? e curva de retenção de água, 3-Tensiômetro e Tanque Classe ?A?) com três nÃveis de cobertura do solo pela palhada de Brachiaria decumbens (1-Solo totalmente coberto, 2-Solo com 50% de cobertura, 3-Solo totalmente descoberto). O delineamento experimental foi em parcelas subdividida com três repetições. Não houve diferença significativa entre os métodos de irrigação e interação. Entretanto, foi verificada diferença significativa (P<0,05) para cobertura, na qual, solos com a superfÃcie totalmente coberta mostraram maior eficiência de uso de água. Devido à palha na superfÃcie do solo a lâmina e o número de irrigações foram menores no solo coberto em relação ao descoberto
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