5,779 research outputs found
Accurate molecular polarizabilities with coupled-cluster theory and machine learning
The molecular polarizability describes the tendency of a molecule to deform
or polarize in response to an applied electric field. As such, this quantity
governs key intra- and inter-molecular interactions such as induction and
dispersion, plays a key role in determining the spectroscopic signatures of
molecules, and is an essential ingredient in polarizable force fields and other
empirical models for collective interactions. Compared to other ground-state
properties, an accurate and reliable prediction of the molecular polarizability
is considerably more difficult as this response quantity is quite sensitive to
the description of the underlying molecular electronic structure. In this work,
we present state-of-the-art quantum mechanical calculations of the static
dipole polarizability tensors of 7,211 small organic molecules computed using
linear-response coupled-cluster singles and doubles theory (LR-CCSD). Using a
symmetry-adapted machine-learning based approach, we demonstrate that it is
possible to predict the molecular polarizability with LR-CCSD accuracy at a
negligible computational cost. The employed model is quite robust and
transferable, yielding molecular polarizabilities for a diverse set of 52
larger molecules (which includes challenging conjugated systems, carbohydrates,
small drugs, amino acids, nucleobases, and hydrocarbon isomers) at an accuracy
that exceeds that of hybrid density functional theory (DFT). The atom-centered
decomposition implicit in our machine-learning approach offers some insight
into the shortcomings of DFT in the prediction of this fundamental quantity of
interest
Characteristic-based formulation of boundary conditions for preconditioned or non-preconditioned flow equations
A numerical implementation of characteristic boundary conditions for Euler and Navier-Stokes equations is presented. The method combines the specified boundary conditions and the outgoing characteristic variables according to the wave propagation directions. In the general case, the proposed boundary conditions update the primitive variables by solving a small system of linear equations (4×4 in 2D, 5×5 in 3D) at each boundary point/cell. The method can be used for both preconditioned and non-preconditioned equations. For a perfect gas without preconditioning, a closed analytical solution is provided. Two possible
methods of extrapolating the outgoing characteristic variables are discussed. Finally, the numerical approach is validated for the
2D internal flow in a channel with a bump
A fault/failure detection algorithm based on a grid of virtual sensors for engine health monitoring
A network of virtual sensors from a set of N real sensor is derived by using a model reduction approach. Starting from the output of a real sensor, analytical correlations are derived an used for linking the corresponding set of virtual sensors. By applying this procedure to all real sensors, a matrix of sensors is derived. The system can monitor its health by comparing the sensor outputs and diagnose if a failure is occurring. The application to a sensor system for a jet engine configuration is illustrated
Antiferromagnetism in the Exact Ground State of the Half Filled Hubbard Model on the Complete-Bipartite Graph
As a prototype model of antiferromagnetism, we propose a repulsive Hubbard
Hamiltonian defined on a graph \L={\cal A}\cup{\cal B} with and bonds connecting any element of with all the
elements of . Since all the hopping matrix elements associated with
each bond are equal, the model is invariant under an arbitrary permutation of
the -sites and/or of the -sites. This is the Hubbard model
defined on the so called -complete-bipartite graph,
() being the number of elements in (). In this
paper we analytically find the {\it exact} ground state for at
half filling for any ; the repulsion has a maximum at a critical
-dependent value of the on-site Hubbard . The wave function and the
energy of the unique, singlet ground state assume a particularly elegant form
for N \ra \inf. We also calculate the spin-spin correlation function and show
that the ground state exhibits an antiferromagnetic order for any non-zero
even in the thermodynamic limit. We are aware of no previous explicit analytic
example of an antiferromagnetic ground state in a Hubbard-like model of
itinerant electrons. The kinetic term induces non-trivial correlations among
the particles and an antiparallel spin configuration in the two sublattices
comes to be energetically favoured at zero Temperature. On the other hand, if
the thermodynamic limit is taken and then zero Temperature is approached, a
paramagnetic behavior results. The thermodynamic limit does not commute with
the zero-Temperature limit, and this fact can be made explicit by the analytic
solutions.Comment: 19 pages, 5 figures .ep
Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua
During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space. It is reasonable to expect that the same happened for planetary systems other than our own. Detection of such interstellar objects would allow us to probe the planetesimal formation processes around other stars, possibly together with the effects of long-term exposure to the interstellar medium. 1I/2017 U1 ‘Oumuamua is the first known interstellar object, discovered by the Pan-STARRS1 telescope in October 2017. The discovery epoch photometry implies a highly elongated body with radii of ~ 200 × 20 m when a comet-like geometric albedo of 0.04 is assumed. The observable interstellar object population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity of 'Oumuamua implies a lack of surface ice. Here, we report spectroscopic characterization of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. We show that this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. An internal icy composition cannot therefore be ruled out by the lack of activity, even though ‘Oumuamua passed within 0.25 au of the Sun
History and recent advancements of electric propulsion and integrated electrical power systems for commercial & naval vessels
Due to developments in power electronics, electric machines, energy storage and control, electric propulsion and integrated electrical power systems have become major trends for commercial and naval vessels. This is mainly due to the fact that the use of electric propulsion and integrated power systems can improve efficiency and fuel consumption while reducing noise and vibration when compared to conventional systems. Such advantages are extremely attractive to vessel owners due to increasingly stringent emission requirements, especially in environmental control areas, from the international maritime organization. This paper aims to summarize the recent advancement of marine power systems including propulsion systems, electrical distribution systems and novel loads
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