12,668 research outputs found
Reliable inference for complex models by discriminative composite likelihood estimation
Composite likelihood estimation has an important role in the analysis of
multivariate data for which the full likelihood function is intractable. An
important issue in composite likelihood inference is the choice of the weights
associated with lower-dimensional data sub-sets, since the presence of
incompatible sub-models can deteriorate the accuracy of the resulting
estimator. In this paper, we introduce a new approach for simultaneous
parameter estimation by tilting, or re-weighting, each sub-likelihood component
called discriminative composite likelihood estimation (D-McLE). The
data-adaptive weights maximize the composite likelihood function, subject to
moving a given distance from uniform weights; then, the resulting weights can
be used to rank lower-dimensional likelihoods in terms of their influence in
the composite likelihood function. Our analytical findings and numerical
examples support the stability of the resulting estimator compared to
estimators constructed using standard composition strategies based on uniform
weights. The properties of the new method are illustrated through simulated
data and real spatial data on multivariate precipitation extremes.Comment: 29 pages, 4 figure
Aziridinium lead iodide: a stable, low bandgap hybrid halide perovskite for photovoltaics
The low ionization energy of an site molecule is a very important factor,
which determines the thermodynamical stability of PbI hybrid halide
perovskites, while the size of the molecule governs the stable phase at room
temperature and, eventually, the bandgap. It is challenging to achieve both a
low ionization energy and the reasonable size for the PbI cage to
circumvent the stability issue inherent to hybrid halide perovskites. Here we
propose a new three-membered charged ring radical, which demonstrates a low
ionization energy that renders a good stability for its corresponding
perovskite and a reasonable cation size that translates into a suitable bandgap
for the photovoltaic application. We use ab initio calculations to evaluate a
polymorphism of the crystal structure of the proposed halide hybrid perovskite,
its stability and electronic properties in comparison to the mainstream
perovskites, such as the methylammonium and formamidinium lead iodide. Our
results highlight the importance of van der Waals interactions for predicting a
correct polymorphism of the perovskite vs hexagonal crystal structure.Comment: 22 pages, 5 figures, 5 table
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