46,222 research outputs found
Implementation of the LDA+U method using the full potential linearized augmented plane wave basis
We provide a straightforward and efficient procedure to combine LDA+U total
energy functional with the full potential linearized augmented plane wave
method. A detailed derivation of the LDA+U Kohn-Sham type equations is
presented for the augmented plane wave basis set, and a simple
``second-variation'' based procedure for self-consistent LDA+U calculations is
given. The method is applied to calculate electronic structure and magnetic
properties of NiO and Gd. The magnetic moments and band eigenvalues obtained
are in very good quantitative agreement with previous full potential LMTO
calculations. We point out that LDA+U reduces the total d charge on Ni by 0.1
in NiO
Radio interferometric gain calibration as a complex optimization problem
Recent developments in optimization theory have extended some traditional
algorithms for least-squares optimization of real-valued functions
(Gauss-Newton, Levenberg-Marquardt, etc.) into the domain of complex functions
of a complex variable. This employs a formalism called the Wirtinger
derivative, and derives a full-complex Jacobian counterpart to the conventional
real Jacobian. We apply these developments to the problem of radio
interferometric gain calibration, and show how the general complex Jacobian
formalism, when combined with conventional optimization approaches, yields a
whole new family of calibration algorithms, including those for the polarized
and direction-dependent gain regime. We further extend the Wirtinger calculus
to an operator-based matrix calculus for describing the polarized calibration
regime. Using approximate matrix inversion results in computationally efficient
implementations; we show that some recently proposed calibration algorithms
such as StefCal and peeling can be understood as special cases of this, and
place them in the context of the general formalism. Finally, we present an
implementation and some applied results of CohJones, another specialized
direction-dependent calibration algorithm derived from the formalism.Comment: 18 pages; 6 figures; accepted by MNRA
First-principles calculation of x-ray dichroic spectra within the full-potential linearized augmented planewave method: An implementation into the Wien2k code
X-ray absorption and its dependence on the polarization of light is a
powerful tool to investigate the orbital and spin moments of magnetic materials
and their orientation relative to crystalline axes. Here, we present a program
for the calculation of dichroic spectra from first principles. We have
implemented the calculation of x-ray absorption spectra for left and right
circularly polarized light into the Wien2k code. In this package, spin-density
functional theory is applied in an all-electron scheme that allows to describe
both core and valence electrons on the same footing. The matrix elements, which
define the dependence of the photo absorption cross section on the polarization
of light and on the sample magnetization, are computed within the dipole
approximation. Results are presented for the L2,3 and M4,5 egdes of CeFe2 and
compared to experiments
Semi-supervised sequence tagging with bidirectional language models
Pre-trained word embeddings learned from unlabeled text have become a
standard component of neural network architectures for NLP tasks. However, in
most cases, the recurrent network that operates on word-level representations
to produce context sensitive representations is trained on relatively little
labeled data. In this paper, we demonstrate a general semi-supervised approach
for adding pre- trained context embeddings from bidirectional language models
to NLP systems and apply it to sequence labeling tasks. We evaluate our model
on two standard datasets for named entity recognition (NER) and chunking, and
in both cases achieve state of the art results, surpassing previous systems
that use other forms of transfer or joint learning with additional labeled data
and task specific gazetteers.Comment: To appear in ACL 201
A penalty method for PDE-constrained optimization in inverse problems
Many inverse and parameter estimation problems can be written as
PDE-constrained optimization problems. The goal, then, is to infer the
parameters, typically coefficients of the PDE, from partial measurements of the
solutions of the PDE for several right-hand-sides. Such PDE-constrained
problems can be solved by finding a stationary point of the Lagrangian, which
entails simultaneously updating the paramaters and the (adjoint) state
variables. For large-scale problems, such an all-at-once approach is not
feasible as it requires storing all the state variables. In this case one
usually resorts to a reduced approach where the constraints are explicitly
eliminated (at each iteration) by solving the PDEs. These two approaches, and
variations thereof, are the main workhorses for solving PDE-constrained
optimization problems arising from inverse problems. In this paper, we present
an alternative method that aims to combine the advantages of both approaches.
Our method is based on a quadratic penalty formulation of the constrained
optimization problem. By eliminating the state variable, we develop an
efficient algorithm that has roughly the same computational complexity as the
conventional reduced approach while exploiting a larger search space. Numerical
results show that this method indeed reduces some of the non-linearity of the
problem and is less sensitive the initial iterate
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