340 research outputs found
Edge Effects in Finite Elongated Graphene Nanoribbons
We analyze the relevance of finite-size effects to the electronic structure
of long graphene nanoribbons using a divide and conquer density functional
approach. We find that for hydrogen terminated graphene nanoribbons most of the
physical features appearing in the density of states of an infinite graphene
nanoribbon are recovered at a length of 40 nm. Nevertheless, even for the
longest systems considered (72 nm long) pronounced edge effects appear in the
vicinity of the Fermi energy. The weight of these edge states scales inversely
with the length of the ribbon and they are expected to become negligible only
at ribbons lengths of the order of micrometers. Our results indicate that
careful consideration of finite-size and edge effects should be applied when
designing new nanoelectronic devices based on graphene nanoribbons. These
conclusions are expected to hold for other one-dimensional systems such as
carbon nanotubes, conducting polymers, and DNA molecules.Comment: 4 pages, 4 figure
Enhanced Half-Metallicity in Edge-Oxidized Zigzag Graphene Nanoribbons
We present a novel comprehensive first-principles theoretical study of the
electronic properties and relative stabilities of edge-oxidized zigzag graphene
nanoribbons. The oxidation schemes considered include hydroxyl, carboxyl,
ether, and ketone groups. Using screened exchange density functional theory, we
show that these oxidized ribbons are more stable than hydrogen-terminated
nanoribbons except for the case of the etheric groups. The stable oxidized
configurations maintain a spin-polarized ground state with antiferromagnetic
ordering localized at the edges, similar to the fully hydrogenated
counterparts. More important, edge oxidation is found to lower the onset
electric field required to induce half-metallic behavior and extend the overall
field range at which the systems remain half-metallic. Once the half-metallic
state is reached, further increase of the external electric field intensity
produces a rapid decrease in the spin magnetization up to a point where the
magnetization is quenched completely. Finally, we find that oxygen containing
edge groups have a minor effect on the energy difference between the
antiferromagnetic ground state and the above-lying ferromagnetic state.Comment: 5 pages,5 figures, 1 tabl
Magnetic Exchange Couplings from Noncollinear Spin Density Functional Perturbation Theory
We propose a method for the evaluation of magnetic exchange couplings based
on noncollinear spin-density functional calculations. The method employs the
second derivative of the total Kohn-Sham energy of a single reference state, in
contrast to approximations based on Kohn-Sham total energy differences. The
advantage of our approach is twofold: It provides a physically motivated
picture of the transition from a low-spin to a high-spin state, and it utilizes
a perturbation scheme for the evaluation of magnetic exchange couplings. The
latter simplifies the way these parameters are predicted using
first-principles: It avoids the non-trivial search for different spin-states
that needs to be carried out in energy difference methods and it opens the
possibility of "black-boxifying" the extraction of exchange couplings from
density functional theory calculations. We present proof of concept
calculations of magnetic exchange couplings in the H--He--H model system and in
an oxovanadium bimetallic complex where the results can be intuitively
rationalized.Comment: J.Chem. Phys. (accepted
Constrained multivariate association with longitudinal phenotypes
The incorporation of longitudinal data into genetic epidemiological studies has the potential to provide valuable information regarding the effect of time on complex disease etiology. Yet, the majority of research focuses on variables collected from a single time point. This aim of this study was to test for main effects on a quantitative trait across time points using a constrained maximum-likelihood measured genotype approach. This method simultaneously accounts for all repeat measurements of a phenotype in families. We applied this method to systolic blood pressure (SBP) measurements from three time points using the Genetic Analysis Workshop 19 (GAW19) whole-genome sequence family simulated data set and 200 simulated replicates. Data consisted of 849 individuals from 20 extended Mexican American pedigrees. Comparisons were made among 3 statistical approaches: (a) constrained, where the effect of a variant or gene region on the mean trait value was constrained to be equal across all measurements; (b) unconstrained, where the variant or gene region effect was estimated separately for each time point; and (c) the average SBP measurement from three time points. These approaches were run for nine genetic variants with known effect sizes (\u3e0.001) for SBP variability and a known gene-centric kernel (MAP4)-based test under the GAW19 simulation model across 200 replicates
Algoritmos paralelos para la resolución de ecuaciones diferenciales ordinarias mediante OpenMP
Proceeding of: XIV Jornadas de Paralelismo, Leganés, Madrid (Spain), 15-17, septiembre, 2003En los últimos años está adquiriendo un gran auge el estudio de los métodos numéricos para la resolución Ecuaciones Diferenciales Ordinarias (ODE’s). Muchos de los métodos numéricos existentes se basan en la aproximación de un modelo continuo mediante un modelo discreto y el cálculo de una solución aproximada en un conjunto finito de puntos. En [1] se presenta una nueva aproximación al cálculo de ODE’s donde la principal aportación pasa por permitir una solución del problema independientemente de que el Jacobiano sea o no invertible. En el presente trabajo se presenta un nuevo algoritmo basado en [1] que permite la resolución de ODE’s. Además, se ha llevado a cabo una implementación paralela sobre arquitecturas de memoria compartida de dicho algoritmo. Tanto el algoritmo secuencial como el algoritmo paralelo desarrollado se han implementado utilizando librerÃas estándar tanto en el cómputo como en la comunicación en aras de obtener portabilidad, robustez y eficiencia
Family Dynamics and Personal Strengths among Dementia Caregivers in Argentina
This study examined whether healthier family dynamics were associated with higher personal strengths of resilience, sense of coherence, and optimism among dementia caregivers in Argentina. Caregivers are usually required to assist individuals with dementia, and family members have typically fulfilled that role. Personal strengths such as resilience, sense of coherence, and optimism have been shown to protect caregivers from some of the negative experiences of providing care, though the family-related variables associated with these personal strengths are largely unknown. Hierarchical multiple regressions investigated the extent to which family dynamics variables are associated with each of the caregiver personal strengths after controlling for demographic and caregiver characteristics. A sample of 105 caregivers from Argentina completed a set of questionnaires during a neurologist visit. Family dynamics explained 32% of the variance in resilience and 39% of the variance in sense of coherence. Greater family empathy and decreased family problems were uniquely associated with higher resilience. Greater communication and decreased family problems were uniquely associated with higher sense of coherence. Optimism was not found to be significantly associated with family dynamics. These results suggest that caregiver intervention research focused on the family may help improve caregiver personal strengths in Argentina and other Latin American countries
Identificación y comparación de indicadores clÃnicos, nutricionales y laborales que influyen en la salud de los trabajadores, por puesto de trabajo en una procesadora de alimentos
Comparative Density Functional Theory Study of Magnetic Exchange Coupling in Di-nuclear Transition Metal Complexes
Multi-center transition metal complexes (MCTMs) with magnetically interacting
ions have been proposed as components for information processing devices and
storage units. For any practical application of MCTMs as magnetic units, it is
crucial to characterize their magnetic behavior, and in particular the
isotropic magnetic exchange coupling, J, between its magnetic centers. Due to
the large size of typical MCTMs, density functional theory (DFT) is the only
practical electronic structure method for evaluating the J coupling. Here we
assess the accuracy of different density functional approximations for
predicting the magnetic couplings of seven di-metal transition metal complexes
with known reliable experimental J couplings spanning from ferromagnetic to
strong antiferromagnetic. The density functionals considered include global
hybrid functionals which mix semilocal density functional approximations and
exact exchange with a fixed admixing parameter, six local hybrid functionals
where the admixing parameters are extended to be spatially dependent, the SCAN
and rSCAN meta-generalized gradient approximations (GGAs), and two widely
used GGAs. We found that global hybrids have a tendency to over-correct the
error in magnetic coupling parameters from the Perdew-Burke-Ernzerhof (PBE)
GGA, while the performance of local hybrid density functionals is scattered
without a clear trend, suggesting that more efforts are needed for the
extension from global to local hybrid density functionals for this particular
property. The SCAN and rSCAN meta-GGAs are found to perform as well or
better than the global and local hybrids on most tested complexes. We further
analyze the charge density redistribution of meta-GGAs as well as global and
local hybrid density functionals with respect to that of PBE, in connection to
the self-interaction error (SIE) or delocalization error
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