77 research outputs found
Ab initio many-body calculations on infinite carbon and boron-nitrogen chains
In this paper we report first-principles calculations on the ground-state
electronic structure of two infinite one-dimensional systems: (a) a chain of
carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield
results were obtained using the restricted Hartree-Fock approach, while the
many-body effects were taken into account by second-order M{\o}ller-Plesset
perturbation theory and the coupled-cluster approach. The calculations were
performed using 6-31 basis sets, including the d-type polarization
functions. Both at the Hartree-Fock (HF) and the correlated levels we find that
the infinite carbon chain exhibits bond alternation with alternating single and
triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In
addition, we also performed density-functional-theory-based local density
approximation (LDA) calculations on the infinite carbon chain using the same
basis set. Our LDA results, in contradiction to our HF and correlated results,
predict a very small bond alternation. Based upon our LDA results for the
carbon chain, which are in agreement with an earlier LDA calculation
calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488
(1998).], we conclude that the LDA significantly underestimates Peierls
distortion. This emphasizes that the inclusion of many-particle effects is very
important for the correct description of Peierls distortion in one-dimensional
systems.Comment: 3 figures (included). To appear in Phys. Rev.
Surface state atoms and their contribution to the surface tension of quantum liquids
We investigate the new type of excitations on the surface of liquid helium.
These excitations, called surfons, appear because helium atoms have discrete
energy level at the liquid surface, being attracted to the surface by the van
der Waals force and repulsed at a hard-core interatomic distance. The
concentration of the surfons increases with temperature. The surfons propagate
along the surface and form a two-dimensional gas. Basing on the simple model of
the surfon microscopic structure, we estimate the surfon activation energy and
effective mass for both helium isotopes. We also calculate the contribution of
the surfons to the temperature dependence of the surface tension. This
contribution explains the great and long-standing discrepancy between theory
and experiment on this temperature dependence in both helium isotopes. The
achieved agreement between our theory and experiment is extremely high. The
comparison with experiment allows to extract the surfon activation energy and
effective mass. The values of these surfon microscopic parameters are in a
reasonable agreement with the calculated from the proposed simple model of
surfon structure.Comment: 10 pages, 6 figure
Native and exotic pests of Eucalyptus: A worldwide perspective.
Eucalyptus species, native to Australia, Indonesia, the Philippines, and New Guinea, are the most widely planted hardwood timber species in the world. The trees, moved around the globe as seeds, escaped the diverse community of herbivores found in their native range. However, a number of herbivore species from the native range of eucalypts have invaded many Eucalyptus-growing regions in North America, Europe, Africa, Asia, and South America in the last 30 years. In addition, there have been shifts of native species, particularly in Africa, Asia, and South America, onto Eucalyptus. There are risks that these species as well as generalist herbivores from other parts of the world will invade Australia and threaten the trees in their native range. The risk to Eucalyptus plantations in Australia is further compounded by planting commercially important species outside their endemic range and shifting of local herbivore populations onto new host trees. Understanding the mechanisms underlying host specificity of Australian insects can provide insight into patterns of host range expansion of both native and exotic insects
A global review of protected area budgets and staff
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Characterization of inhibitors and substrates of Anopheles gambiae CYP6Z2
Three CYP6Z genes are linked to a major pyrethroid resistance locus in the mosquito Anopheles gambiae. We have expressed CYP6Z2 in Escherichia coli and produced a structural model in order to examine its role in detoxification. E. coli membranes co-expressing CYP6Z2 and An. gambiae P450 reductase (AgCPR) catalysed the dealkylation of benzyloxyresorufin with kinetic parameters K(m) = 0.13 microM; K(cat) = 1.5 min(-1). The IC(50) values of a wide range of compounds were measured. Pyrethroids cypermethrin and permethrin produced low IC(50) values, but were not metabolized. Plant flavanoids were the most potent inhibitors. Several compounds were shown to be substrates, suggesting that CYP6Z2 has broad substrate specificity and plays an important chemo-protective role during the herbivorous phase of the life-cycle
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