7,455 research outputs found
Local correlation functional for electrons in two dimensions
We derive a local approximation for the correlation energy in two-dimensional
electronic systems. In the derivation we follow the scheme originally developed
by Colle and Salvetti for three dimensions, and consider a Gaussian
approximation for the pair density. Then, we introduce an ad-hoc modification
which better accounts for both the long-range correlation, and the
kinetic-energy contribution to the correlation energy. The resulting functional
is local, and depends parametrically on the number of electrons in the system.
We apply this functional to the homogeneous electron gas and to a set of
two-dimensional quantum dots covering a wide range of electron densities and
thus various amounts of correlation. In all test cases we find an excellent
agreement between our results and the exact correlation energies. Our
correlation functional has a form that is simple and straightforward to
implement, but broadly outperforms the commonly used local-density
approximation
Density-based mixing parameter for hybrid functionals
A very popular ab-initio scheme to calculate electronic properties in solids
is the use of hybrid functionals in density functional theory (DFT) that mixes
a portion of Fock exchange with DFT functionals. In spite of their success, a
major problem still remains, related to the use of one single mixing parameter
for all materials. Guided by physical arguments that connect the mixing
parameter to the dielectric properties of the solid, and ultimately to its band
gap, we propose a method to calculate this parameter from the electronic
density alone. This method is able to cut significantly the error of
traditional hybrid functionals for large and small gap materials, while
retaining a good description of structural properties. Moreover, its
implementation is simple and leads to a negligible increase of the
computational time.Comment: submitte
Coleta de germoplas de jaborandi Pilocarpus microphyllus Stapf. nos estados do Maranhão e Pará.
The Ultrasensitivity of Living Polymers
Synthetic and biological living polymers are self-assembling chains whose
chain length distributions (CLDs) are dynamic. We show these dynamics are
ultrasensitive: even a small perturbation (e.g. temperature jump) non-linearly
distorts the CLD, eliminating or massively augmenting short chains. The origin
is fast relaxation of mass variables (mean chain length, monomer concentration)
which perturbs CLD shape variables before these can relax via slow chain growth
rate fluctuations. Viscosity relaxation predictions agree with experiments on
the best-studied synthetic system, alpha-methylstyrene.Comment: 4 pages, submitted to Phys. Rev. Let
Investigating interaction-induced chaos using time-dependent density functional theory
Systems whose underlying classical dynamics are chaotic exhibit signatures of
the chaos in their quantum mechanics. We investigate the possibility of using
time-dependent density functional theory (TDDFT) to study the case when chaos
is induced by electron-interaction alone. Nearest-neighbour level-spacing
statistics are in principle exactly and directly accessible from TDDFT. We
discuss how the TDDFT linear response procedure can reveal the mechanism of
chaos induced by electron-interaction alone. A simple model of a two-electron
quantum dot highlights the necessity to go beyond the adiabatic approximation
in TDDFT.Comment: 8 pages, 4 figure
Trigo-mourisco na alimentação de frangos de corte.
Trigo-mourisco (Fagopyrum esculentum, Moench) em rações de frangos de corte. Linhagem Hubbard. Delineamento experimental inteiramente casualizado
Sistema agroflorestal em área de pequeno produtor na região do Tapajós, Estado do Pará: avaliação após doze anos de implantado.
bitstream/item/63497/1/Oriental-Doc99.PD
Structure, growth and production of a remarkably abundant population of the common goby, Pomatoschistus microps (Actinopterygii: Gobiidae)
The common goby, Pomatoschistus microps,
is a relevant species from estuarine food webs, playing
important roles as predator of polychaetes and crustaceans
and as prey for larger fishes and crustaceans. The
Minho estuary (NW Portugal) is a relatively wellpreserved
and productive system. To assess the population
structure and production of P. microps in this
estuary, monthly samples were undertaken in three
different areas along an estuarine gradient in the lower
estuary. The density of P. microps varied considerably
among seasons and sampling stations, with higher densities
occurring in summer and autumn. The lowest
densities were found closer to the sea. In general, the
density of females was higher than the density of males
in all sampling stations, while juveniles were more
abundant within a salt marsh area. Compared with other
European estuaries, our data showed a remarkable
higher density and production values of P. microps.
This may be related to the high freshwater input and
the low salinities found in this estuary. In addition, we
hypothesize that the lower density of the sympatric
species P. minutus and the high availability of bivalve
shells observed in the Minho estuary may have also
contributed to the present results, once P. minutus and P.
microps often display a diet overlap and the bivalve
shells are crucial for the common goby reproduction.This research was supported by a grant from Iceland, Liechtenstein and Norway
through the EEA Financial Mechanism and the Norwegian
Financial Mechanism. A.T. Souza has a PhD grant (SFRH/BD/
71232/2010) from the Fundação para a Ciência e a Tecnologia
(Portugal). I. Martins was financed for CIÊNCIA 2007 and the
project INPACTAR PTDC/MAR/111537/2009; FCT; COMPETE;
QREN; UE
Efeitos do estresse salino na germinação, emergência e estabelecimento da plântula de cajueiro anão precoce.
Objetivou-se com este trabalho avaliar os efeitos da salinidade em três estádios de desenvolvimento (ED) da plântula de cajueiro anão-precoce, a saber, germinação, emergência e estabelecimento da plântula, tentando estabelecer uma relação entre o desenvolvimento e o acúmulo de íons nos tecidos da plântula. Para isso, castanhas do clone CCP-06 de cajueiro anão precoce foram semeadas em bandejas de plástico contendo vermiculita umedecida com água destilada (0,0 dS m-1) ou soluções de NaCl com condutividades elétricas variando entre 3,0 e 18,0 dS m-1 e mantidas em casa de vegetação. Determinaram-se a percentagem de plântulas que alcançaram os ED estudados e o tempo e a velocidade com que isso ocorreu, assim como a matéria seca dos cotilédones e eixo embrionário e os teores de Na+, K+ e Cl- do eixo embrionário. A salinidade retardou o desenvolvimento das plântulas nos ED correspondentes à emergência e estabelecimento, entretanto apenas neste último ED é que a percentagem final de plântulas foi reduzida, acompanhada pela inibição da depleção das reservas cotiledonares e decréscimo da massa seca do eixo embrionário. Em todos os ED, os teores de Na+ e Cl- aumentaram com a salinidade, mas isso foi mais pronunciado nas plântulas estabelecidas, nas quais houve redução dos teores de K+. Os resultados obtidos sugerem que as plântulas de cajueiro anão precoce são mais sensíveis à salinidade durante a etapa de estabelecimento e que os efeitos deletérios desse estresse são mediados, pelo menos em parte, pelo acúmulo excessivo de íons Na+ e Cl- na plântula
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