5,207 research outputs found
Troubleshooting Time-Dependent Density-Functional Theory for Photochemical Applications: Oxirane
The development of analytic-gradient methodology for excited states within
conventional time-dependent density-functional theory (TDDFT) would seem to
offer a relatively inexpensive alternative to better established
quantum-chemical approaches for the modeling of photochemical reactions.
However, even though TDDFT is formally exact, practical calculations involve
the use of approximate functionals, in particular the TDDFT adiabatic
approximation, whose use in photochemical applications must be further
validated. Here, we investigate the prototypical case of the symmetric CC ring
opening of oxirane. We demonstrate by direct comparison with the results of
high-quality quantum Monte Carlo calculations that, far from being an
approximation on TDDFT, the Tamm-Dancoff approximation (TDA) is a practical
necessity for avoiding triplet instabilities and singlet near instabilities,
thus helping maintain energetically reasonable excited-state potential energy
surfaces during bond breaking. Other difficulties one would encounter in
modeling oxirane photodynamics are pointed out but none of these is likely to
prevent a qualitatively correct TDDFT/TDA description of photochemistry in this
prototypical molecule.Comment: 19 pages, 17 figures, submitted to the Journal of Chemical Physic
Simultaneous Contralateral Vestibular Schwannoma and Middle Ear Paraganglioma Tumor
To the best of our knowledge, only 2 cases of a simultaneous contralateral vestibular schwannoma (VS) and middle ear paraganglioma (MEP) have previously been reported in literature. We report the third case observed in a 43-year-old male, who presented with an 11-year history of right-sided hearing loss and a 1-year history of left-sided pulsatile tinnitus. A magnetic resonance imaging (MRI) showed a VS on the right side and computer tomography (CT) identified a Fisch type A1 paraganglioma on the left side. The VS was treated using a translabyrinthine approach and the MEP was kept under radiological observation for 1 year. Due to the growth of the MEP (Fisch type A2), it was treated with excision via a retroauricular approach. Our case was very challenging because there was a different and important pathology on each side, both carrying a risk of deafness as a consequence of the disease and/or the treatments
Rock properties of the upper-crust in Central Apennines (Italy) derived from high-resolution 3-D tomography
High-resolution 3-D P and S-wave velocity models of a central sector of the Apennines (Central Italy) are computed by inverting first arrival times from an aftershock sequence (September–December, 1997) following the Mw 5.7 and Mw 6.0 Umbria-Marche earthquakes that occurred on September 26, 1997. The high quality of the data set, especially for the S-wave, allows us to compute 3-D variations in Vp, Vp/Vs and Vp · Vs. The anomalies can be interpreted as lateral changes in rock type and fracturing, which control fluid diffusion and variation in pore pressure. This is in agreement with a poro-elastic view that can be inferred from the spatio-temporal evolution of the seismic sequence
Dark matter from the scalar sector of 3-3-1 models without exotic electric charges
We show that three SU(2) singlet neutral scalars (two CP-even and one CP-odd)
in the spectrum of models based on the gauge symmetry SU(3)_c X SU(3)_L X
U(1)_X, which do not contain exotic electric charges, are realistic candidates
for thermally generated self-interacting dark matter in the Universe, a type of
dark matter that has been recently proposed in order to overcome some
difficulties of collisionless cold dark matter models at the galactic scale.
These candidates arise without introducing a new mass scale in the model and/or
without the need for a discrete symmetry to stabilize them, but at the expense
of tuning several combinations of parameters of the scalar potential.Comment: RevTeX, 11 pages. v2: typos corrected, one reference added. v3:
clarifications added, four more references added. To appear in Europhys. Let
Energy and variance optimization of many body wave functions
We present a simple, robust and efficient method for varying the parameters
in a many-body wave function to optimize the expectation value of the energy.
The effectiveness of the method is demonstrated by optimizing the parameters in
flexible Jastrow factors, that include 3-body electron-electron-nucleus
correlation terms, for the NO and decapentaene (CH)
molecules. The basic idea is to add terms to the straightforward expression for
the Hessian that are zero when the integrals are performed exactly, but that
cancel much of the statistical fluctuations for a finite Monte Carlo sample.
The method is compared to what is currently the most popular method for
optimizing many-body wave functions, namely minimization of the variance of the
local energy. The most efficient wave function is obtained by optimizing a
linear combination of the energy and the variance.Comment: 4 pages, 4 figures, minor corrections of inexact statements, missing
Local structure of REFeAsO (RE=La, Pr, Nd, Sm) oxypnictides studied by Fe K-edge EXAFS
Local structure of REOFeAs (RE=La, Pr, Nd, Sm) system has been studied as a
function of chemical pressure varied due to different rare-earth size. Fe
K-edge extended X-ray absorption fine structure (EXAFS) measurements in the
fluorescence mode has permitted to compare systematically the inter-atomic
distances and their mean square relative displacements (MSRD). We find that the
Fe-As bond length and the corresponding MSRD hardly show any change, suggesting
the strongly covalent nature of this bond, while the Fe-Fe and Fe-RE bond
lengths decrease with decreasing rare earth size. The results provide important
information on the atomic correlations that could have direct implication on
the superconductivity and magnetism of REOFeAs system, with the chemical
pressure being a key ingredient
Isotope effect on the E2g phonon and mesoscopic phase separation near the electronic topological transition in Mg1-xAlxB2
We report the boron isotope effect on the E2g phonon mode by micro-Raman
spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes
10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the
full range decreasing near x = 0. The intraband electron-phonon (e-ph)
coupling, for the electrons in the sigma band, has been extracted from the E2g
line-width and frequency softening. Tuning the Fermi energy near the electronic
topological transition (ETT), where the sigma Fermi surface changes from 2D to
3D topology the E2g mode, shows the known Kohn anomaly on the 2D side of the
ETT and a splitting of the E2g phonon frequency into a hard and soft component
from x = 0 to x = 0.28. The results suggest a minor role of the intraband
phonon mediated pairing in the control of the high critical temperature in
Mg1-xAlxB2. The common physical features of diborides with the novel multigap
FeAs-based superconductors and cuprates is discussed.Comment: 19 pages, 6 figure
Evidence of an advantage in visuo-spatial memory for bilingual compared to monolingual speakers
Previous research has indicated that bilinguals outperform monolinguals in cognitive tasks involving spatial working memory. The present study examines evidence for this claim using a different and arguably more ecologically valid method (the change blindness task). Bilingual and monolingual participants were presented with two versions of the same scenes and required to press a key as soon as they identified the alteration. They also completed the word and alpha span tasks, and the Corsi blocks task. The results in the change blindness task, controlled for group differences in non-verbal reasoning, indicated that bilinguals were faster and more accurate than monolinguals at detecting visual changes. Similar group differences were found on the Corsi block task. Unlike previous findings, no group differences were found on the verbal memory tasks. The results are discussed with reference to mechanisms of cognitive control as a locus of transfer between bilingualism and spatial working memory tasks
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