1,819 research outputs found
Stacking of oligo and polythiophenes cations in solution: surface tension and dielectric saturation
The stacking of positively charged (or doped) terthiophene oligomers and
quaterthiophene polymers in solution is investigated applying a recently
developed unified electrostatic and cavitation model for first-principles
calculations in a continuum solvent. The thermodynamic and structural patterns
of the dimerization are explored in different solvents, and the distinctive
roles of polarity and surface tension are characterized and analyzed.
Interestingly, we discover a saturation in the stabilization effect of the
dielectric screening that takes place at rather small values of .
Moreover, we address the interactions in trimers of terthiophene cations, with
the aim of generalizing the results obtained for the dimers to the case of
higher-order stacks and nanoaggregates
Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations
We present a novel approach that allows to calculate the dielectric response
of periodic systems in the quantum Monte Carlo formalism. We employ a many-body
generalization for the electric enthalpy functional, where the coupling with
the field is expressed via the Berry-phase formulation for the macroscopic
polarization. A self-consistent local Hamiltonian then determines the
ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo
calculations where the polarization's fixed point is estimated from the average
on an iterative sequence, sampled via forward-walking. This approach has been
validated for the case of an isolated hydrogen atom, and then applied to a
periodic system, to calculate the dielectric susceptibility of
molecular-hydrogen chains. The results found are in excellent agreement with
the best estimates obtained from the extrapolation of quantum-chemistry
calculations.Comment: 5 page 2figure
Population and Size Distribution of Small Jovian Trojan Asteroids
We present a study of Jovian Trojan objects detected serendipitously during
the course of a sky survey conducted at the University of Hawaii 2.2-meter
telescope. We used a 8192 x 8192 pixel charge-coupled device (CCD) mosaic to
observe 20 deg^2 at locations spread over the L4 Lagrangian swarm and reached a
limiting magnitude V = 22.5 mag (50% of maximum detection efficiency).
Ninety-three Jovian Trojans were detected with radii 2 - 20 km (assumed albedo
0.04). Their differential magnitude distribution has a slope of 0.40 +/- 0.05
corresponding to a power law size distribution index 3.0 +/- 0.3 (1-sigma). The
total number of L4 Trojans with radii > 1 km is of order 1.6 x 10^5 and their
combined mass (dominated by the largest objects) is ~ 10^{-4} M_{Earth}. The
bias-corrected mean inclination is 13.7 +/- 0.5 deg. We also discuss the size
and spatial distribution of the L4 swarm.Comment: 21 pages, 11 figures. AJ, in pres
Simulation of Heme using DFT+U: a step toward accurate spin-state energetics
We investigate the DFT+U approach as a viable solution to describe the
low-lying states of ligated and unligated iron heme complexes. Besides their
central role in organometallic chemistry, these compounds represent a
paradigmatic case where LDA, GGA, and common hybrid functionals fail to
reproduce the experimental magnetic splittings. In particular, the imidazole
pentacoordinated heme is incorrectly described as a triplet by all usual DFT
flavors. In this study we show that a U parameter close to 4 eV leads to spin
transitions and molecular geometries in quantitative agreement with
experiments, and that DFT+U represents an appealing tool in the description of
iron porphyrin complexes, at a much reduced cost compared to correlated
quantum-chemistry methods. The possibility of obtaining the U parameter from
first-principles is explored through a self-consistent linear-response
formulation. We find that this approach, which proved to be successful in other
iron systems, produces in this case some overestimation with respect to the
optimal values of U.Comment: To be published in The Journal of Physical Chemistry B 30 pages, 15
figure
Taxonomy of asteroid families among the Jupiter Trojans: Comparison between spectroscopic data and the Sloan Digital Sky Survey colors
We present a comparative analysis of the spectral slope and color
distributions of Jupiter Trojans, with particular attention to asteroid
families. We use a sample of data from the Moving Object Catalogue of the Sloan
Digital Sky Survey, together with spectra obtained from several surveys. A
first sample of 349 observations, corresponding to 250 Trojan asteroids, were
extracted from the Sloan Digital Sky Survey, and we also extracted from the
literature a second sample of 91 spectra, corresponding to 71 Trojans. The
spectral slopes were computed by means of a least-squares fit to a straight
line of the fluxes obtained from the Sloan observations in the first sample,
and of the rebinned spectra in the second sample. In both cases the reflectance
fluxes/spectra were renormalized to 1 at 6230 . We found that the
distribution of spectral slopes among Trojan asteroids shows a bimodality.
About 2/3 of the objects have reddish slopes compatible with D-type asteroids,
while the remaining bodies show less reddish colors compatible with the P-type
and C-type classifications. The members of asteroid families also show a
bimodal distribution with a very slight predominance of D-type asteroids, but
the background is clearly dominated by the D-types. The L4 and L5 swarms show
different distributions of spectral slopes, and bimodality is only observed in
L4. These differences can be attributed to the asteroid families since the
backgraound asteroids show the same slope distribtuions in both swarms. The
analysis of individual families indicates that the families in L5 are
taxonomically homogeneous, but in L4 they show a mixture of taxonomic types. We
discuss a few scenarios that might help to interpret these results.Comment: 20 pages, 15 figures, 2 table
Transition state method and Wannier functions
We propose a computational scheme for materials where standard Local Density
Approximation (LDA) fails to produce a satisfactory description of excitation
energies. The method uses Slater's "transition state" approximation and Wannier
functions basis set. We define a correction to LDA functional in such a way
that its variation produces one-electron energies for Wannier functions equal
to the energies obtained in "transition state" constrained LDA calculations. In
the result eigenvalues of the proposed functional could be interpreted as
excitation energies of the system under consideration. The method was applied
to MgO, Si, NiO and BaBiO and gave an improved agreement with experimental
data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl
Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
Transition-metal centers are the active sites for many biological and
inorganic chemical reactions. Notwithstanding this central importance,
density-functional theory calculations based on generalized-gradient
approximations often fail to describe energetics, multiplet structures,
reaction barriers, and geometries around the active sites. We suggest here an
alternative approach, derived from the Hubbard U correction to solid-state
problems, that provides an excellent agreement with correlated-electron quantum
chemistry calculations in test cases that range from the ground state of Fe
and Fe to the addition-elimination of molecular hydrogen on FeO. The
Hubbard U is determined with a novel self-consistent procedure based on a
linear-response approach.Comment: 5 pages, 3 figures, Phys. Rev. Lett., in pres
Effect of rotational disruption on the size-frequency distribution of the Main Belt asteroid population
The size distribution of small asteroids in the Main Belt is assumed to be
determined by an equilibrium between the creation of new bodies out of the
impact debris of larger asteroids and the destruction of small asteroids by
collisions with smaller projectiles. However, for a diameter less than 6 km we
find that YORP-induced rotational disruption significantly contributes to the
erosion even exceeding the effects of collisional fragmentation. Including this
additional grinding mechanism in a collision evolution model for the asteroid
belt, we generate size-frequency distributions from either an accretional
(Weidenschilling, 2011) or an "Asteroids were born big" (Morbidelli, 2009)
initial size-frequency distribution that are consistent with observations
reported in Gladman et al. (2009). Rotational disruption is a new mechanism
that must be included in all future collisional evolution models of asteroids.Comment: 5 pages, 3 figures, accepted in MNRAS letter
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