Time filter

Source Type

Crdenas C.,University of Chile | Crdenas C.,McMaster University | Ayers P.W.,McMaster University | Cedillo A.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics

Density-functional-theory-based chemical reactivity indicators are formulated for degenerate and near-degenerate ground states. For degenerate states, the functional derivatives of the energy with respect to the external potential do not exist, and must be replaced by the weaker concept of functional variation. The resultant reactivity indicators depend on the specific perturbation. Because it is sometimes impractical to compute reactivity indicators for a specific perturbation, we consider two special cases: point-charge perturbations and Dirac delta function perturbations. The Dirac delta function perturbations provide upper bounds on the chemical reactivity. Reactivity indicators using the common used average of degenerate states approximation for degenerate states provide a lower bound on the chemical reactivity. Unfortunately, this lower bound is often extremely weak. Approximate formulas for the reactivity indicators within the frontier-molecular-orbital approximation and special cases (two or three degenerate spatial orbitals) are presented in the supplementary material. One remarkable feature that arises in the frontier molecular orbital approximation, and presumably also in the exact theory, is that removing electrons sometimes causes the electron density to increase at the location of a negative (attractive) Dirac delta function perturbation. That is, the energetic response to a reduction in the external potential can increase even when the number of electrons decreases. © 2011 American Institute of Physics. Source

Salcedo J.,Instituto Politcnico Nacional | Salcedo B.,Instituto Tecnolgico Autnomo Of Mxico | Snchez-Bernabe F.J.,University Autnoma Metropolitana Iztapalapa
Computers and Mathematics with Applications

In this paper we introduce a method for the numerical solutions of initial value problems, that combines finite differences with Simpson's rule. The effectiveness of the method is proved by solving, in one spatial dimension, a stiff and convection-dominated transport problem. To solve the same problem in two spatial dimensions, the proposed method was used successfully in combination with Strang's operator decomposition method. © 2010 Elsevier Ltd. All rights reserved. Source

Vazquez-Luna A.,University Autnoma Metropolitana Iztapalapa | Rivera-Cabrera F.,University Autnoma Metropolitana Iztapalapa | Perez-Flores L.J.,University Autnoma Metropolitana Iztapalapa | Diaz-Sobac R.,University of Veracruz
Journal of Economic Entomology

The effect of the use of rootstock Criollo on the susceptibility of Manila mango fruit to infestation by Anastrepha obliqua (Macquart) was determined in the present investigation. Growth, quality parameters (Soluble solids content (%), citric acid content (%), firmness, monoterpene volatiles (3-carene, limonene, α-pinene, and β-myrcene), main flavonoids content (milligrams/100 g fresh pulp), and larvicidal activity of methanolic extracts, as well as the degree of infestation during preharvest development of the fruits were analyzed. The results indicated that the rootstock did not have any significant effect on growth, soluble solids content, or citric acid content; although it increased firmness as well as 3-carene levels and main flavonoids content, resulting in a greater resistance to infestation by A. obliqua. The obtained results support the use of this rootstock because of the beneficial effects observed on the resistance of mango fruit to infestation by this fruit fly. © 2011 Entomological Society of America. Source

Ramrez Trejo M.D.R.,University Autnoma Metropolitana Iztapalapa | Perez-Garca B.,University Autnoma Metropolitana Iztapalapa | Perez-Salicrup D.R.,University of Morelia | Orozco-Segovia A.,National University of Costa Rica
Journal of Tropical Ecology

Pteridium caudatum is a fern that frequently invades burnt areas in the Yucatn Peninsula and other neotropical sites. While post-fire regeneration of this fern apparently occurs mainly by vegetative means, little is known about the role of its spores in post-fire regeneration and in colonization of newly invaded fields. Central to these questions is whether bracken fern spores maintain their viability after fires. Here we experimentally evaluate the effect of fire-induced temperatures on Pteridium caudatum spore germination. We used 1200-cm3 blocks containing a constant fuel load of 47.4 g of litter, in which we placed spores at three different depths. The blocks were then ignited, and temperatures at each depth were monitored at 1-min intervals for 2 h. One day after the experimental fires, spores were dug out and cultured at 25 C and 12-h light/dark cycles. Soil temperatures decreased significantly in relation to depth during fires. Spores on the surface were severely affected by fire, while those buried at 1 and 3 cm showed 77% germination. Germination in unburned controls was 86%. Our results suggest that during fires, Pteridium caudatum spores buried a few centimetres below the surface have a high percentage of viability, which could explain the rapid establishment of this species in burnt fields. Copyright © 2010 Cambridge University Press. Source

Valdes-Parada F.J.,University Autnoma Metropolitana Iztapalapa | Alvarez-Ramrez J.,University Autnoma Metropolitana Iztapalapa
Journal of Chemical Physics

Asymmetric diffusion has been observed in different contexts, from transport in stratified and fractured porous media to diffusion of ions and macromolecular solutes through channels in biological membranes. Experimental and numerical observations have shown that diffusion is facilitated in the direction of positive void fraction (i.e., porosity) gradients. This work uses the method of volume averaging in order to obtain effective medium equations for systems with void fraction gradients for passive and diffusive mass transport processes. The effective diffusivity is computed from the solution of an associated closure problem in representative unit cells that allow considering porosity gradients. In this way, the results in this work corroborate previous findings showing that the effective diffusivity exhibits important directional asymmetries for geometries with void fraction gradients. Numerical examples for simple geometries (a section with an obstacle and a channel with varying cross section) show that the diffusion asymmetry depends strongly on the system configuration. The magnitude of this dependence can be quantified from the results in this work. © 2011 American Institute of Physics. Source

Discover hidden collaborations