4,846 research outputs found
Is protein folding problem really a NP-complete one ? First investigations
To determine the 3D conformation of proteins is a necessity to understand
their functions or interactions with other molecules. It is commonly admitted
that, when proteins fold from their primary linear structures to their final 3D
conformations, they tend to choose the ones that minimize their free energy. To
find the 3D conformation of a protein knowing its amino acid sequence,
bioinformaticians use various models of different resolutions and artificial
intelligence tools, as the protein folding prediction problem is a NP complete
one. More precisely, to determine the backbone structure of the protein using
the low resolution models (2D HP square and 3D HP cubic), by finding the
conformation that minimize free energy, is intractable exactly. Both the proof
of NP-completeness and the 2D prediction consider that acceptable conformations
have to satisfy a self-avoiding walk (SAW) requirement, as two different amino
acids cannot occupy a same position in the lattice. It is shown in this
document that the SAW requirement considered when proving NP-completeness is
different from the SAW requirement used in various prediction programs, and
that they are different from the real biological requirement. Indeed, the proof
of NP completeness and the predictions in silico consider conformations that
are not possible in practice. Consequences of this fact are investigated in
this research work.Comment: Submitted to Journal of Bioinformatics and Computational Biology,
under revie
Finite volume analysis of reinforced concrete structure cracking using a thermo-plastic-damage model
This paper proposes modifications to the phenomenological model formulation called CDPM2, developed by Grassl et al. [1]. The proposed modifications are designed to enhance model performance with coupling to temperature effects. A very strong coupling between nonlinear elasticity, plasticity, nonlocal damage evolution and temperature gradient is used to simulate arbitrary crack propagation. The use of FVM to model solid damage is a numerical challenge. This approach presents some advantages such as: ensuring that discretization is conservative even when the geometry is changing; providing a simple formulation that can be obtained directly from a difference method; and employing unstructured meshes. Most authors have neglected the nonlinearity of concrete in the elastic domain from the start of loading to the plastic domain. In this paper we confirm that concrete rheology is not linear even under low loading. Also, since the so-called fracture energy is a key parameter needed to determine the size of cracks and how they propagate in space, we consider that the fracture energy is both material and geometrical parameter dependent. For this reason, we developed a new approach which includes adaptive mesh, nonlinear rheology and thermal effects to re-calculate fracture energy at each time step. Many authors use a constant value obtained from experiments to calculate fracture energy; others use a numerical correlation. In this study, the fracture energy parameter is not constant and can vary with temperature or/and with a change in geometry due to concrete failure. As is well known, the mesh quality of complex geometries is very important for making accurate predictions. A new meshing tool was developed using the C++ programming language. This tool is faster, more accurate and produces a high-quality structured mesh. The predictions obtained were compared to a wide variety of experimental data and showed good agreement
Dynamics of electrons in the quantum Hall bubble phases
In Landau levels N > 1, the ground state of the two-dimensional electron gas
(2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for
small filling of the partially filled Landau level, into a succession of bubble
states with increasing number of guiding centers per bubble as the filling
increases, to a modulated stripe state near half filling. In this work, we show
that these first-order phase transitions between the bubble states lead to
measurable discontinuities in several physical quantities such as the density
of states and the magnetization of the 2DEG. We discuss in detail the behavior
of the collective excitations of the bubble states and show that their spectra
have higher-energy modes besides the pinned phonon mode. The frequencies of
these modes, at small wavevector k, have a discontinuous evolution as a
function of filling factor that should be measurable in, for example, microwave
absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40
Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems
In higher Landau levels (N>0) and around filling factors nu =4N+1, a
two-dimensional electron gas in a double-quantum-well system supports a stripe
groundstate in which the electron density in each well is spatially modulated.
When a parallel magnetic field is added in the plane of the wells, tunneling
between the wells acts as a spatially rotating effective Zeeman field coupled
to the ``pseudospins'' describing the well index of the electron states. For
small parallel fields, these pseudospins follow this rotation, but at larger
fields they do not, and a commensurate-incommensurate transition results.
Working in the Hartree-Fock approximation, we show that the combination of
stripes and commensuration in this system leads to a very rich phase diagram.
The parallel magnetic field is responsible for oscillations in the tunneling
matrix element that induce a complex sequence of transitions between
commensurate and incommensurate liquid or stripe states. The homogeneous and
stripe states we find can be distinguished by their collective excitations and
tunneling I-V, which we compute within the time-dependent Hartree-Fock
approximation.Comment: 23 pages including 8 eps figure
Solitonic Excitations in Linearly Coherent Channels of Bilayer Quantum Hall Stripes
In some range of interlayer distances, the ground state of the
two-dimensional electron gas at filling factor nu =4N+1 with N=0,1,2,... is a
coherent stripe phase in the Hartree-Fock approximation. This phase has
one-dimensional coherent channels that support charged excitations in the form
of pseudospin solitons. In this work, we compute the transport gap of the
coherent striped phase due to the creation of soliton-antisoliton pairs using a
supercell microscopic unrestricted Hartree-Fock approach. We study this gap as
a function of interlayer distance and tunneling amplitude. Our calculations
confirm that the soliton-antisoliton excitation energy is lower than the
corresponding Hartree-Fock electron-hole pair energy. We compare our results
with estimates of the transport gap obtained from a field-theoretic model valid
in the limit of slowly varying pseudospin textures.Comment: 15 pages, 8 figure
Rubidium Rydberg macrodimers
We explore long-range interactions between two atoms excited into high
principal quantum number n Rydberg states, and present calculated potential
energy surfaces (PES) for various symmetries of doubly excited ns and np
rubidium atoms. We show that the PES for these symmetries exhibit deep (~GHz)
potential wells, which can support very extended (~micrometers) bound
vibrational states (macrodimers). We present n-scaling relations for both the
depth De of the wells and the equilibrium separations Re of these macrodimers,
and explore their response to small electric fields and stability with respect
to predissociation. Finally, we present a scheme to form and study these
macrodimers via photoassociation, and show how one can probe the various
\ell-character of the potential wells
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