Descripción: Fil:Durán, G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: This work describes simple decompositions of the energy of molecular systems according to schemes that partition the three-dimensional space. The components of those decompositions depend on one and two atomic domains thus providing a meaningful chemical information about the nature of different bondings among the atoms which compose the system. Our algorithms can be applied at any level of theory (correlated or uncorrelated wave functions). The results reported here, obtained at the Hartree-Fock level in selected molecules, show a good agreement with the chemical picture of molecules and require a low computational cost in comparison with other previously reported decompositions.

Descripción: This paper describes a matrix formulation for the correlated hole theory within the framework of the domain-averaged model in many electron systems (atoms, molecules, condensed matter, etc.). General relationships between this quantity and one-particle reduced density matrices for any independent particle or correlated state functions are presented. This formulation turns out to be suitable for computational purposes due to the straightforward introduction of cumulants of two-particle reduced density matrices within the quantum field structure. Numerical calculations in selected simple molecular systems have been performed in order to determine preliminary correlated values for such a quantity.

Descripción: A computational scheme was developed for a fourth-rank hyprmagnetizability tensor denoted by Xαβγδ. It was shown that this instrinsic property of diamagnetic atoms and molecules was useful to rationalize their nonlinear response to intense magnetic field. The terms connected with the fourth power of the perturbing field which represent the fourth-rank hypermagnetizability of the H2, HF, H2O, NH3, and CH4 molecules were evaluated at the coupled Hartree-Fock level of accuracy. The Gaugeless basis sets of increasing size and flexibility that adopted two different coordinate systems to estimate the degree of convergence of theoretical tensor components was employed.

Descripción: We study the problem of counting the total number of affine solutions of a system of n binomials in n variables over an algebraically closed field of characteristic zero. We show that we may decide in polynomial time if that number is finite. We give a combinatorial formula for computing the total number of affine solutions (with or without multiplicity) from which we deduce that this counting problem is # P-complete. We discuss special cases in which this formula may be computed in polynomial time; in particular, this is true for generic exponent vectors. © 2006 Elsevier Inc. All rights reserved.

Descripción: The resolution of molecular electric dipole polarizabilities into atomic contributions was studied in the context of methyl and ethyl derivatives, CH3-X and CH3-CH2-X. The reliability of the partitioning schemes allowing for adhoc quantum chemical tools was also tested. Results depicted a good degree of the transferability from molecule to molecule as shown by the evaluation of gross atomic isotropic contributions for carbon, hydrogen, and heteroatoms.

Descripción: This paper presents a new generalization of the graph multicoloring problem. We propose a Branch-and-Cut algorithm based on a new integer programming formulation. The cuts used are valid inequalities that we could identify to the polytope associated with the model. The Branch-and-Cut system includes separation heuristics for the valid inequalities, specific initial and primal heuristics, branching and pruning rules. We report on computational experience with random instances. © 2009 Elsevier B.V. All rights reserved.

Descripción: Computational procedures, based on (i) the Ramsey common origin approach and (ii) the continuous transformation of the origin of the quantum mechanical current density-diamagnetic zero (CTOCD-DZ), were applied at the Hartree-Fock level to determine electric quadrupole polarizabilities of nuclear magnetic shielding for molecules in the presence of a nonuniform electric field with a uniform gradient. The quadrupole polarizabilities depend on the origin of the coordinate system, but values of the magnetic field induced at a reference nucleus, determined via the CTOCD-DZ approach, are origin independent for any calculations relying on the algebraic approximation, irrespective of size and quality of the (gaugeless) basis set employed. On the other hand, theoretical estimates of the induced magnetic field obtained by single-origin methods are translationally invariant only in the limit of complete basis sets. Calculations of electric quadrupole polarizabilities of nuclear magnetic shielding are reported for H2, HF, H2 O, N H3, and C H4 molecules. © 2008 American Institute of Physics.

Descripción: Continuum solvent models have become a standard technique in the context of electronic structure calculations, yet no implementations have been reported capable to perform molecular dynamics at solid-liquid interfaces. We propose here such a continuum approach in a density functional theory framework using plane-wave basis sets and periodic boundary conditions. Our work stems from a recent model designed for Car-Parrinello simulations of quantum solutes in a dielectric medium [D. A. Scherlis, J. Chem. Phys. 124, 074103 (2006)], for which the permittivity of the solvent is defined as a function of the electronic density of the solute. This strategy turns out to be inadequate for systems extended in two dimensions: the dependence of the dielectric function on the electronic density introduces a new term in the Kohn-Sham potential, which becomes unphysically large at the interfacial region, seriously affecting the convergence of the self-consistent calculations. If the dielectric medium is properly redefined as a function of the atomic coordinates, a good convergence is obtained and the constant of motion is conserved during the molecular dynamics simulations. The Poisson problem is solved using a multigrid method, and in this way Car-Parrinello molecular dynamics simulations of solid-liquid interfaces can be performed at a very moderate computational cost. This scheme is employed to investigate the acid-base equilibrium at the TiO2 -water interface. The aqueous behavior of titania surfaces has stimulated a large amount of experimental research, but many open questions remain concerning the molecular mechanisms determining the chemistry of the interface. Here we make an attempt to answer some of them, putting to the test our continuum model. © 2009 American Institute of Physics.

Descripción: The advanced spaceborne thermal emission and reflection radiometer (ASTER) was used to identify different deposits of hydrothermal alteration which indicates that several important lithological groups can be mapped in areas with good exposure by using spectral-matching techniques. Different methods are tested in order to identify and map zones with hydrothermal alteration minerals using the ASTER dataset. These areas are often referred to having large quantities of clay minerals which can be detected using multispectral imagery. Several authors have developed different procedures to map these hydrothermal minerals. Among the simplest ones, band combinations and band ratios have proven to be very useful tools for identifying targets. Lithology indexes point to reinforce the spectral response of this group of minerals working with band products and ratios. So far, these techniques do not need a full image correction. Other methods here employed require further processing of the ASTER scene, especially when spectral data are used. These techniques include spectral angle mapper (SAM) classification and minimum noise fraction (MNF) transforms to segregate noise and reduce computational requirements. Spectral data used in this paper were collected from field samples using SWIR (short wave infrared) reflectance spectroscopy and derived from the scene itself. These mapping methods have been tested in areas of known hydrothermal alteration occurrences, e.g. Los Pelambres, El Pachón and Altar, and in other sector of Santa Cruz region like Carnicerias and La Coipa; all of these at the south westernmost part of San Juan province. The result of this work is here presented as a series of images showing lithology indexes and an expected mineral assembly.

Descripción: Background: Alzheimer's disease (AD) is characterized by a neurodegenerative progression that alters cognition. On a phenotypical level, cognition is evaluated by means of the MiniMental State Examination (MMSE) and the post-morten examination of Neurofibrillary Tangle count (NFT) helps to confirm an AD diagnostic. The MMSE evaluates different aspects of cognition including orientation, short-term memory (retention and recall), attention and language. As there is a normal cognitive decline with aging, and death is the final state on which NFT can be counted, the identification of brain gene expression biomarkers from these phenotypical measures has been elusive. Methodology/Principal Findings: We have reanalysed a microarray dataset contributed in 2004 by Blalock et al. of 31 samples corresponding to hippocampus gene expression from 22 AD subjects of varying degree of severity and 9 controls. Instead of only relying on correlations of gene expression with the associated MMSE and NFT measures, and by using modern bioinformatics methods based on information theory and combinatorial optimization, we uncovered a 1,372-probe gene expression signature that presents a high-consensus with established markers of progression in AD. The signature reveals alterations in calcium, insulin, phosphatidylinositol and wnt-signalling. Among the most correlated gene probes with AD severity we found those linked to synaptic function, neurofilament bundle assembly and neuronal plasticity. Conclusions/Significance: A transcription factors analysis of 1,372-probe signature reveals significant associations with the EGR/KROX family of proteins, MAZ, and E2F1. The gene homologous of EGR1, zif268, Egr-1 or Zenk, together with other members of the EGR family, are consolidating a key role in the neuronal plasticity in the brain. These results indicate a degree of commonality between putative genes involved in AD and prion-induced neurodegenerative processes that warrants further investigation. © 2010 Go ́mez Ravetti et al.