7,901 research outputs found
Constrained Dynamics for Quantum Mechanics I. Restricting a Particle to a Surface
We analyze constrained quantum systems where the dynamics do not preserve the
constraints. This is done in particular for the restriction of a quantum
particle in Euclidean n-space to a curved submanifold, and we propose a method
of constraining and dynamics adjustment which produces the right Hamiltonian on
the submanifold when tested on known examples. This method we hope will become
the germ of a full Dirac algorithm for quantum constraints. We take a first
step in generalising it to the situation where the constraint is a general
selfadjoint operator with some additional structures.Comment: 49 pages, TEX, input files amssym.def, amssym.te
Gibberella musae (Fusarium musae) sp. nov., a recently discovered species from banana is sister to F. verticillioides
Several strains of Fusarium isolated from banana were identified previously as F. verticillioides (Sacc.) Nirenberg but described as unable to produce fumonisin. Here we report biochemical and morphological evidence, as well as multilocus phylogenetic analyses based on elongation factor (EF-1a), calmodulin, b-tubulin, and the second largest subunit of RNA polymerase II (RPB2) sequences, indicating that these isolates represent a unique lineage in the Gibberella fujikuroi species complex related to but distinct from F. verticillioides. Together with previous results of molecular studies, as well as with results of metabolite analyses, crossing experiments, pathogenicity tests and morphological characterization, these new data indicate that these strains isolated from banana represent a new species, Gibberella musae Van Hove et al. sp. nov. (anamorph: Fusarium musae Van Hove et al. sp. nov.), which is described herei
Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips
Geometric configuration and energy of a hydrogen molecule centered between
two point-shaped tips of equal charge are calculated with the variational
quantum Monte-Carlo (QMC) method without the restriction of the
Born-Oppenheimer (BO) approximation. Ground state nuclear distribution,
stability, and low vibrational excitation are investigated. Ground state
results predict significant deviations from the BO treatment that is based on a
potential energy surface (PES) obtained with the same QMC accuracy. The quantum
mechanical distribution of molecular axis direction and bond length at a
sub-nanometer level is fundamental for understanding nanomechanical dynamics
with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry
yields a uniform azimuthal distribution of the molecular axis vector relative
to the tip-tip axis. With approaching tips towards each other, the QMC sampling
shows an increasing loss of spherical symmetry with the molecular axis still
uniformly distributed over the azimuthal angle but peaked at the tip-tip
direction for negative tip charge while peaked at the equatorial plane for
positive charge. This directional behavior can be switched between both stable
configurations by changing the sign of the tip charge and by controlling the
tip-tip distance. This suggests an application in the field of molecular
machines.Comment: 20 pages, 10 figure
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
Intramolecular Torque, an Indicator of the Internal Rotation Direction of Rotor Molecules and Similar Systems
Torque is ubiquitous in many molecular systems, including collisions,
chemical reactions, vibrations, electronic excitations and especially rotor
molecules. We present a straightforward theoretical method based on forces
acting on atoms and obtained from atomistic quantum mechanics calculations, to
quickly and qualitatively determine whether a molecule or sub-unit thereof has
a tendency to rotation and, if so, around which axis and in which sense:
clockwise or counterclockwise. The method also indicates which atoms, if any,
are predominant in causing the rotation. Our computational approach can in
general efficiently provide insights into the rotational ability of many
molecules and help to theoretically screen or modify them in advance of
experiments or before analyzing their rotational behavior in more detail with
more extensive computations guided by the results from the torque approach. As
an example, we demonstrate the effectiveness of the approach using a specific
light-driven molecular rotary motor which was successfully synthesized and
analyzed in prior experiments and simulations.Comment: 11 pages, 4 figures, 1 SI fil
Variation in sequence and location of the fumonisin mycotoxin niosynthetic gene cluster in Fusarium
In Fusarium, the ability to produce fumonisins is governed by a 17-gene fumonisin biosynthetic gene (FUM) cluster. Here, we examined the cluster in F. oxysporum strain O-1890 and nine other species selected to represent a wide range of the genetic diversity within the GFSC
Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5
We have measured polarized spectra of Raman scattering, infrared and optical
transmission of NaV_2O_5 single crystals above the temperature of the
spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines
are strongly asymmetric, due to the spin-phonon interaction. In addition to the
phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the
E||a FIR transmission spectrum. A possible origin of these bands is discussed.
The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d
electronic transitions while the absorption edge above 3 eV is supposed to
correspond to the onset of charge-transfer transitions.Comment: 7 figures, 8 page
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