1,545 research outputs found
Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: A First-Principles Quantum Dynamical Study
We report on high-dimensional quantum dynamical simulations of
torsion-induced exciton migration in a single-chain oligothiophene segment
comprising twenty repeat units, using a first-principles parametrized Frenkel
J-aggregate Hamiltonian. Starting from an initial inter-ring torsional defect,
these simulations provide evidence of an ultrafast two-time scale process at
low temperatures, involving exciton-polaron formation within tens of
femtoseconds, followed by torsional relaxation on a ~300 femtosecond time
scale. The second step is the driving force for exciton migration, as initial
conjugation breaks are removed by dynamical planarization. The quantum coherent
nature of the elementary exciton migration step is consistent with experimental
observations highlighting the correlated and vibrationally coherent nature of
the dynamics on ultrafast time scales.Comment: 4 pages, 4 figure
Hartree-Fock Many-Body Perturbation Theory for Nuclear Ground-States
We investigate the order-by-order convergence behavior of many-body
perturbation theory (MBPT) as a simple and efficient tool to approximate the
ground-state energy of closed-shell nuclei. To address the convergence
properties directly, we explore perturbative corrections up to 30th order and
highlight the role of the partitioning for convergence. The use of a simple
Hartree-Fock solution to construct the unperturbed basis leads to a convergent
MBPT series for soft interactions, in contrast to, e.g., a harmonic oscillator
basis. For larger model spaces and heavier nuclei, where a direct high-order
MBPT calculation in not feasible, we perform third-order calculation and
compare to advanced ab initio coupled-cluster calculations for the same
interactions and model spaces. We demonstrate that third-order MBPT provides
ground-state energies for nuclei up into tin isotopic chain that are in
excellent agreement with the best available coupled-cluster results at a
fraction of the computational cost.Comment: 6 pages, 5 figures, 1 tabl
Ab Initio Path to Heavy Nuclei
We present the first ab initio calculations of nuclear ground states up into
the domain of heavy nuclei, spanning the range from 16-O to 132-Sn based on
two- plus three-nucleon interactions derived within chiral effective field
theory. We employ the similarity renormalization group for preparing the
Hamiltonian and use coupled-cluster theory to solve the many-body problem for
nuclei with closed sub-shells. Through an analysis of theoretical uncertainties
resulting from various truncations in this framework, we identify and eliminate
the technical hurdles that previously inhibited the step beyond medium-mass
nuclei, allowing for reliable validations of nuclear Hamiltonians in the heavy
regime. Following this path we show that chiral Hamiltonians qualitatively
reproduce the systematics of nuclear ground-state energies up to the
neutron-rich Sn isotopes.Comment: 5 pages, 5 figure
Understanding and Comparing Deep Neural Networks for Age and Gender Classification
Recently, deep neural networks have demonstrated excellent performances in
recognizing the age and gender on human face images. However, these models were
applied in a black-box manner with no information provided about which facial
features are actually used for prediction and how these features depend on
image preprocessing, model initialization and architecture choice. We present a
study investigating these different effects.
In detail, our work compares four popular neural network architectures,
studies the effect of pretraining, evaluates the robustness of the considered
alignment preprocessings via cross-method test set swapping and intuitively
visualizes the model's prediction strategies in given preprocessing conditions
using the recent Layer-wise Relevance Propagation (LRP) algorithm. Our
evaluations on the challenging Adience benchmark show that suitable parameter
initialization leads to a holistic perception of the input, compensating
artefactual data representations. With a combination of simple preprocessing
steps, we reach state of the art performance in gender recognition.Comment: 8 pages, 5 figures, 5 tables. Presented at ICCV 2017 Workshop: 7th
IEEE International Workshop on Analysis and Modeling of Faces and Gesture
Xenobiotic monooxygenase activity and the response to inducers of cytochrome P-450 during embryonic and larval development in fish
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1981Data demonstrating the presence and inducibility of the xenobiotic
monooxygenase system in fish embryos and larvae are described. The
ontogeny of benzo(a)pyrene monooxygenase (BPM) activity, and NADPH- and
NADH-cytochrome c reductase activities, were followed in microsomes
prepared from whole embryos of the estuarine killifish Fundulus
heteroclitus. BPM activity was detectable as early as 4 days from
fertilization, prior to the appearance of the liver rudiment, which
indicates a substantial role for the extrahepatic tissues in xenobiotic
metabolism in Fundulus embryos. At all stages assayed before hatching,
BPM activity was uniformly low, but within 24 hours of hatching there was
a 10-fold increase in specific activity. This increase was shown not to be age-dependent but
required hatching, and was not an artifact of the
presence of endogenous inhibitors in embryos.
Both NADPH- and NADH-cytochrome c reductase activities were
measurable at all stages assayed. The developmental patterns of these
two reductases were distinct from each other and did not closely
correlate with that of BPM activity. However, the functional involvement
of the NADPH-cytochrome c reductase in monooxygenase activity was
indicated by the inhibition of BPM activity by cytochrome c. The
metabolism of benzo(a)pyrene by fractions prepared from whole Fundulus
embryos and eleutheroembryos appears to be catalyzed by a typical
cytochrome P-450 dependent monooxygenase. This activity is localized in
the microsomal fraction, requires O2, NADPH and native enzyme, and is
inhibited by CO. NADPH supports much higher activity than NADH.
BPM activity was detectable in the livers of Fundulus
eleutheroembryos, larvae and juveniles. The level of activity in
Fundulus eleutheroembryo livers was about 1/4 the average adult
activity. Specific activity rose continuously from the end of the
embryonic period into the juvenile period when adult levels were
approached.
BPM activity was also measurable in the livers of brook trout
(Salvelinus fontinalis) embryos and eleutheroembryos. The ontogenic
pattern contrasted with that seen in Fundulus. At 6 and 1 days before
hatching BPM specific activity in embryonic liver was close to the adult
level. After hatching there was a 3-fold increase in activity, thus the
livers of eleutheroembryos were considerably more active in metabolizing
BP than those of adult brook trout.
BPM activity was inducible in Fundulus embryos by both Aroclor 1254
and No.2 Fuel oii. Embryos were competent to respond to induction as
early as 4 days from fertilization. In Fundulus eleutheroembryos,
Aroclor 1254 induced BPM activity in both the liver and extrahepatic
tissues. Aminopyrine N-demethylase activity was detectable in microsomes
prepared from whole eleutheroembryos, but was not induced by Aroclor
1254. Neither NADPH- nor NADH-cytochrome c reductase activities were
induced by Aroclor 1254 before hatching, but after hatching both
activities were induced.
A striking developmental change in the sensitivty of the induction
response was observed in Fundulus. The tissue levels of PCBs necessary
to produce a maximal induction of BPM activity were at least 5 times
lower in post-hatching stages compared to prehatching stages. The
relative insensitivity of the induction response prior to hatching may
serve to protect embryos from damage from activated metabolites during
organogenesis.
Aroclor 1254 was also shown to induce BPM activity in brook trout
embryonic liver. The data obtained with both Fundulus and brook trout
indicate that levels of PCBs occurring in fish in contaminated
environments are likely to induce the monooxygenase system during
embryonic development.
Metabolites of benzo( a) pyrene produced by microsomes prepared from
adult Fundulus liver, and untreated and PCB exposed eleutheroembryos were
analyzed by HPLC. Similar metabolite profiles were obtained in all
cases, with a high proportion of benzo-ring dihydrodiols. The
dihydrodiol peaks produced by e leutheroembryo microsomes were abolished
by TCPO, indicating the presence of epoxide hydrolase. These results
suggest that Fundulus embryos and eleutheroembryos can activate BP to the
highly mutagenic trans-7, 8-dihydrodiol-9, 10-epoxides. Fish embryonic
monooxygenase activity may play a role in pollutant-induced lesions,
including teratogenic effects, by producing reactive and mutagenic
metabolites during organodifferentiation.This work was supported by National Science Foundation grants OCE77-24517 and OCE80-18569
Ab Initio Calculations of Medium-Mass Nuclei with Explicit Chiral 3N Interactions
We present the first ab initio coupled-cluster calculations of medium-mass
nuclei with explicit chiral three-nucleon (3N) interactions. Using a spherical
formulation of coupled cluster with singles and doubles excitations including
explicit 3N contributions, we study ground states of 16,24-O, 40,48-Ca and
56-Ni. We employ chiral NN plus 3N interactions softened through a similarity
renormalization group (SRG) transformation at the three-body level. We
investigate the impact of all truncations and quantify the resulting
uncertainties---this includes the contributions from triples excitations, the
truncation of the set of three-body matrix elements, and the omission of
SRG-induced four-body interactions. Furthermore, we assess the quality of a
normal-ordering approximation of the 3N interaction beyond light nuclei. Our
study points towards the predictive power of chiral Hamiltonians in the
medium-mass regime.Comment: 6 pages, 3 figures, 2 table
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