4,873 research outputs found
Density functional theory for molecular size consistency and fractional charge
We show that the exact universal functional can be accurately approximated as
a sum of local parts F(rho_x)+(rho_y) for a system made of distantly separated
densities rho_x and rho_y of fractional charge. The derived F has the same form
as the one from the grand-canonical-ensemble treatment. For a molecule that is
made of an external potential that has two distantly separated locales, F can
be applied with an accurate, external-potential-based constraint and achieve
the size consistency. The outer loop in the two-step constrained search
formalism is modified to include a search over the number of electrons in each
locale of the external potential. Components of F can be extracted based on the
Kohn-Sham assumption and with an aid of a model external potential where a
molecule with fractional charge (referred to as fractional molecule) can be
defined as part of a single non-fractional molecule of two-fold degeneracy with
a nondegeneracy condition. We show that the ensemble density of a fractional
molecule is non-degenerate noninteracting wavefunction v-representable. The
noninteracting kinetic energy and the exact exchange energy functionals of such
a density are well defined and have the same forms as those for nonfractional
systems. A correlation functional is defined that pertains to the fractionally
occupied highest occupied molecular orbital only. The exact exchange energy is
discontinuous as the number of electrons passing through an odd integer but its
sum with the new correlation energy is continuous. This sum remains an
upper-bound to the formal Kohn-Sham exchange-correlation energy of the
fractional molecule. It yields the correct result for a well-designed example
of effective fractional occupancies in literature
IL12. Density Functional Theory Method for Nondynamic/Strong Correlation
Nondynamic and strong correlation imposes the major challenge to the current density functional theory (DFT), and counts for the majority of the failures of DFT in a variety of areas such as catalysis, organic open-shell molecules and materials. The problem is often characterized as multireference in nature. In this talk, we will present a density functional model based on single-determinant Kohn-Sham density functional theory [1]. It combines Becke’13 method with a new model for kinetic correlation via adiabatic connection based on physical arguments and some exact conditions for both the weak and strong correlations. The result is a single-term functional for correlation of all strength, and is named as KP16/B13 (Kong-Proynov’16/Becke’13). KP16/B13 is the first model of its kind implemented with self-consistent field. The preliminary results show that the model, with only three empirical parameters, recovers the majority of left-right nondynamic/strong correlation upon bond dissociation and performs well for near equilibrium properties such as heats of formation, singlet−triplet energy splittings of diradicals. It also describes well a linear chain of H atoms with many strongly correlated electrons. The new development offers the hope for efficient computation of systems with multireference in nature. Jing Kong, Middle Tennessee State University Emil Proynov, Middle Tennessee State Universit
Quenching depends on morphologies: implications from the ultraviolet-optical radial color distributions in Green Valley Galaxies
In this Letter, we analyse the radial UV-optical color distributions in a
sample of low redshift green valley (GV) galaxies, with the Galaxy Evolution
Explorer (GALEX)+Sloan Digital Sky Survey (SDSS) images, to investigate how the
residual recent star formation distribute in these galaxies. We find that the
dust-corrected colors of early-type galaxies (ETGs) are flat out to
, while the colors turn blue monotonously when for
late-type galaxies (LTGs). More than a half of the ETGs are blue-cored and have
remarkable positive NUV color gradients, suggesting that their star
formation are centrally concentrated; the rest have flat color distributions
out to . The centrally concentrated star formation activity in a large
portion of ETGs is confirmed by the SDSS spectroscopy, showing that 50 %
ETGs have EW(H) \AA. For the LTGs, 95% of them show uniform
radial color profiles, which can be interpreted as a red bulge plus an extended
blue disk. The links between the two kinds of ETGs, e.g., those objects having
remarkable "blue-cored" and those having flat color gradients, are less known
and require future investigations. It is suggested that the LTGs follow a
general picture that quenching first occur in the core regions, and then
finally extend to the rest of the galaxy. Our results can be re-examined and
have important implications for the IFU surveys, such as MaNGA and SAMI.Comment: ApJ Letter, accepted. Five figure
Multilabel Consensus Classification
In the era of big data, a large amount of noisy and incomplete data can be
collected from multiple sources for prediction tasks. Combining multiple models
or data sources helps to counteract the effects of low data quality and the
bias of any single model or data source, and thus can improve the robustness
and the performance of predictive models. Out of privacy, storage and bandwidth
considerations, in certain circumstances one has to combine the predictions
from multiple models or data sources to obtain the final predictions without
accessing the raw data. Consensus-based prediction combination algorithms are
effective for such situations. However, current research on prediction
combination focuses on the single label setting, where an instance can have one
and only one label. Nonetheless, data nowadays are usually multilabeled, such
that more than one label have to be predicted at the same time. Direct
applications of existing prediction combination methods to multilabel settings
can lead to degenerated performance. In this paper, we address the challenges
of combining predictions from multiple multilabel classifiers and propose two
novel algorithms, MLCM-r (MultiLabel Consensus Maximization for ranking) and
MLCM-a (MLCM for microAUC). These algorithms can capture label correlations
that are common in multilabel classifications, and optimize corresponding
performance metrics. Experimental results on popular multilabel classification
tasks verify the theoretical analysis and effectiveness of the proposed
methods
Density Functional Model for Nondynamic and Strong Correlation
A single-term density functional model for nondynamic and strong correlation
is presented, based on single-determinant Kohn-Sham density functional theory.
It is derived from modeling the adiabatic connection and contains only two
nonlinear empirical parameters. Preliminary tests show that the model recovers
majority of nondynamic correlation during a molecular dissociation and at the
same time performs reasonably for atomization energies. It demonstrates the
feasibility of developing DFT functionals for nondynamic and strong correlation
within the single-determinant KS scheme.Comment: Journal of Chemical Theory and Computation, 201
The UV-optical colours of brightest cluster galaxies in optically and X-ray selected clusters
Many brightest cluster galaxies (BCGs) at the centers of X-ray selected
clusters exhibit clear evidence for recent star formation. However, studies of
BCGs in optically-selected clusters show that star formation is not enhanced
when compared to control samples of non-BCGs of similar stellar mass. Here we
analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected
clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in
X-ray selected clusters. We convolve the SDSS images of the BCGs to match the
resolution of the GALEX data and we measure UV-optical colours in their inner
and outer regions. We find that optically-selected BCGs exhibit smaller scatter
in optical colours and redder inner NUV-r colours than the control galaxies,
indicating that they are a homogenous population with very little ongoing star
formation. The BCGs in the X-ray selected cluster sample span a similar range
in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs,
those located in clusters with central cooling times of less than 1 Gyr are
significantly bluer than those located in clusters where the central gas
cooling times are long. Our main conclusion is that the location of a galaxy at
the centre of its halo is not sufficient to determine whether or not it is
currently forming stars. One must also have information about the thermodynamic
state of the gas in the core of the halo.Comment: 14 pages, 13 figures, Accepted by MNRA
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