46,342 research outputs found
Nuclear shell evolution and in-medium NN interaction
We report on a quantitative study of the evolution of the nuclear shell structure, in particular, effective single-particle energies (ESPEs), based on the spin-tensor decomposition of an effective two-body shell-model interaction. While the global trend of the ESPEs is mainly due to the central term of the effective interaction, variations of shell gaps invoke various components of the in-medium NN force. From a detailed analysis of a well-fitted realistic interaction in the sdpf shell-model space, two most important contributions for the evolution of the N = 20 and N = 28 shell gaps are confirmed to be the central term and the tensor term. The role of the latter is dominant to explain the energy shift of spin-orbit partners. Spin-tensor analysis of microscopic effective interactions in sd, pf, and gds shell-model spaces, contrasted with that of the phenomenologically adjusted ones, shows no evidence of amplification of the tensor component contribution; however, it points toward the neglect of three-body forces in the present microscopic interactions
Modeling reactivity to biological macromolecules with a deep multitask network
Most
small-molecule drug candidates fail before entering the market,
frequently because of unexpected toxicity. Often, toxicity is detected
only late in drug development, because many types of toxicities, especially
idiosyncratic adverse drug reactions (IADRs), are particularly hard
to predict and detect. Moreover, drug-induced liver injury (DILI)
is the most frequent reason drugs are withdrawn from the market and
causes 50% of acute liver failure cases in the United States. A common
mechanism often underlies many types of drug toxicities, including
both DILI and IADRs. Drugs are bioactivated by drug-metabolizing enzymes
into reactive metabolites, which then conjugate to sites in proteins
or DNA to form adducts. DNA adducts are often mutagenic and may alter
the reading and copying of genes and their regulatory elements, causing
gene dysregulation and even triggering cancer. Similarly, protein
adducts can disrupt their normal biological functions and induce harmful
immune responses. Unfortunately, reactive metabolites are not reliably
detected by experiments, and it is also expensive to test drug candidates
for potential to form DNA or protein adducts during the early stages
of drug development. In contrast, computational methods have the potential
to quickly screen for covalent binding potential, thereby flagging
problematic molecules and reducing the total number of necessary experiments.
Here, we train a deep convolution neural networkî—¸the XenoSite
reactivity modelî—¸using literature data to accurately predict
both sites and probability of reactivity for molecules with glutathione,
cyanide, protein, and DNA. On the site level, cross-validated predictions
had area under the curve (AUC) performances of 89.8% for DNA and 94.4%
for protein. Furthermore, the model separated molecules electrophilically
reactive with DNA and protein from nonreactive molecules with cross-validated
AUC performances of 78.7% and 79.8%, respectively. On both the site-
and molecule-level, the model’s performances significantly
outperformed reactivity indices derived from quantum simulations that
are reported in the literature. Moreover, we developed and applied
a selectivity score to assess preferential reactions with the macromolecules
as opposed to the common screening traps. For the entire data set
of 2803 molecules, this approach yielded totals of 257 (9.2%) and
227 (8.1%) molecules predicted to be reactive only with DNA and protein,
respectively, and hence those that would be missed by standard reactivity
screening experiments. Site of reactivity data is an underutilized
resource that can be used to not only predict if molecules are reactive,
but also show where they might be modified to reduce toxicity while
retaining efficacy. The XenoSite reactivity model is available at http://swami.wustl.edu/xenosite/p/reactivity
Cobalt-Porphyrin Catalyzed Electrochemical Reduction of Carbon Dioxide in Water II: Mechanism from First Principles
We apply first principles computational techniques to analyze the
two-electron, multi-step, electrochemical reduction of CO2 to CO in water using
cobalt porphyrin as a catalyst. Density Functional Theory calculations with
hybrid functionals and dielectric continuum solvation are used to determine the
steps at which electrons are added. This information is corroborated with ab
initio molecular dynamics simulations in an explicit aqueous environment which
reveal the critical role of water in stabilizing a key intermediate formed by
CO2 bound to cobalt. Using potential of mean force calculations, the
intermediate is found to spontaneously accept a proton to form a carboxylate
acid group at pH<9.0, and the subsequent cleavage of a C-OH bond to form CO is
exothermic and associated with a small free energy barrier. These predictions
suggest that the proposed reaction mechanism is viable if electron transfer to
the catalyst is sufficiently fast. The variation in cobalt ion charge and spin
states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the
need for computing electrochemical potentials are emphasized.Comment: 33 pages, 7 figure
Keyed Non-Parametric Hypothesis Tests
The recent popularity of machine learning calls for a deeper understanding of
AI security. Amongst the numerous AI threats published so far, poisoning
attacks currently attract considerable attention. In a poisoning attack the
opponent partially tampers the dataset used for learning to mislead the
classifier during the testing phase.
This paper proposes a new protection strategy against poisoning attacks. The
technique relies on a new primitive called keyed non-parametric hypothesis
tests allowing to evaluate under adversarial conditions the training input's
conformance with a previously learned distribution . To do so we
use a secret key unknown to the opponent.
Keyed non-parametric hypothesis tests differs from classical tests in that
the secrecy of prevents the opponent from misleading the keyed test
into concluding that a (significantly) tampered dataset belongs to
.Comment: Paper published in NSS 201
Performance Analysis of Adaptive Notch Filter Active Damping Methods for Grid-Connected Converters under a Varying Grid Impedance
Grid connected converters commonly use LCL filters for harmonic content suppression. However, associated with such filters is a resonant frequency at which the gain value increases significantly. To mitigate this problem, a notch filter is introduced into the current control loop of the converter. When tuned to the LCL resonant frequency, it introduces an opposing notch, thereby neutralizing the resonance effect. To ensure robustness of the control system, the notch filter must be made adaptive. This will ensure any variation in the resonant frequency, either due to a change in grid impedance or aging of components, can be tracked accurately. This paper provides two novel methods of online tuning for the adaptive notch filter using grid impedance estimation and discrete Fourier transform (DFT) techniques. Simulation results show that both methods are capable of fast and accurate detection of the resonant frequency, for varying strengths of the grid
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