Browsing by Author "Fuentealba, P"

Now showing 1 - 7 of 7
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    Basis set superposition error in atomic cluster calculations
    (1999) Fuentealba, P; Simón-Manso, Y
    In this work the basis set superposition error (BSSE) has been analyzed for the systems Cu-2, Cu-3, Cu-6 and Cu-13 as prototypes of metallic clusters. Various basis sets have been tested. In addition, pseudopotentials of 1- and 19-valence electrons have also been studied in relationship to the BSSE. The results are conclusive. At the present, it is almost impossible to do an all-electron calculation in transition-metal clusters without great error in basis set superposition, even for clusters of moderate size. The only solution seems to be using pseudopotentials with a carefully chosen basis set. (C) 1999 Elsevier Science B.V. All rights reserved.
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    Empirical energy-density relationships for the analysis of substituent effects in chemical reactivity
    (2000) Pérez, P; Simón-Manso, Y; Aizman, A; Fuentealba, P; Contreras, R
    Electronic substituent effects may be rationalized in terms of Hammett-like linear relationships between global energy-dependent quantities and local electronic descriptors of reactivity. These linear relationships are framed on a local hard and soft acids and bases (HSAB) principle in accord with previous results reported by Li and Evens [J. Am. Chem Sec. 1995, 117, 7756]. Chemical substitution is indirectly assessed as local responses at the active center of the substrate, with the Fukui function and local softness as the key quantities within the present approach. This model of chemical substitution has a potential advantage with respect to models based on group properties using the electronegativity equalization principle (EEP), since the transferability of group properties is not required. The formalism is illustrated for the gas-phase basicity of alkylamines, and the gas-phase acidity of alkyl alcohols and alkyl thioalcohols. Our results based on the local HSAB rule agree a ell with those obtained from group properties analysis based on the EEP, suggesting that bath empirical rules consistently complement each other.
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    Molecular electronic excitations and the minimum polarizability principle
    (2000) Fuentealba, P; Simón-Manso, Y; Chattaraj, PK
    The validity of the minimum polarizability principle upon electronic excitation is studied as a companion principle of that obtained by Chattaraj and Poddar in the case of the maximum hardness principle. Twelve diatomic molecules have been selected and, both the hardness and the dipole polarizability for the ground and excited states have been calculated by means of ab initio density functional calculations using Sadlej's basis set. It has been found that a molecule is less polarizable in its ground state than in an electronically excited state of the same spin multiplicity.
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    On the condensed Fukui function
    (2000) Fuentealba, P; Pérez, P; Contreras, R
    A critical comparison among recently proposed methods for evaluating the condensed Fukui function neglecting relaxation effects is presented. The sign of the condensed Fukui function is discussed and arguments for a positive definite condensed Fukui function are given. Our numerical calculations in two series of molecules show that: (i) the condensed Fukui function can give, in general, valuable information about the site selectivity in chemical reactions and systematization in a family of molecules. In particular, it has been shown that the selectivity towards protonation in anilines and derivatives molecules can be correctly assessed by the electrophilic Fukui function described in this paper. Within this approach non-negative values for the condensed Fukui function are obtained for the relevant protonation sites in these polyfunctional systems; and (ii) the solvent effects on the condensed Fukui function are negligible, confirming a recently presented theoretical prediction. (C) 2000 American Institute of Physics. [S0021-9606(00)30331-2].
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    On the density functional relationship between static dipole polarizability and global softness
    (1998) Simón-Manso, Y; Fuentealba, P
    The relationship between global softness and static dipole polarizability is explored from the analogy of the spherical averages defined within density functional theory. A cubic relationship is obtained that is consistent with the experimental observation for atoms. The relationship is found to hold reasonably well for similarly bonded molecules.
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    Static dipole polarizabilities through density functional methods
    (1997) Fuentealba, P; Simón-Manso, Y
    Various density functionals have been tested in calculating atomic and molecular dipole polarizabilities. For atoms, it has been found that the results are not competitive with more sophisticated ab initio methods. Exchange and correlation effects have been analyzed separately to show that the main cause of errors lies in the exchange functional models. Strong numerical evidence is given to support the idea that a right asymptotic behavior of the exchange potential is essential to obtain reliable values for the dipole polarizabilities. In this sense, the hybrid method proposed by Becke (J. Chem. Phys. 1993, 98, 5648) and the phenomenological exchange potential proposed by van Leeuwen and Baerends (Phys. Rev. 1994, A49, 2421), performed much better. For molecules, the comparison is more difficult because of the scarcity of reliable experimental values as well as ab initio calculations including correlation effects. The results have however shown that the bonding effects predominate over the asymptotic behavior of the potential.
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    Validity of the minimum polarizability principle in molecular vibrations and internal rotations: An ab initio SCF study
    (AMER CHEMICAL SOC, 1999) Chattaraj, PK; Fuentealba, P; Jaque, P; Toro Labbe, A
    Molecular vibrations in ammonia (NH3) and hydrogen sulfide (H2S), and internal rotations in hydrogen peroxide (HOOH), hydrogen thioperoxide (HSOH), hydrogen persulfide (HSSH), and ethylene (C2H4) are studied using ab initio SCF methods at the Hartree-Fock level using:a-standard Pople 6-311G** basis set. Polarizability values are calculated using both Pople's and Sadlej's basis sets, Any nontotally symmetric distortion in bond length or bond angle along the vibrational symmetry coordinates of a molecule around its equilibrium geometry decreases the equilibrium hardness value and increases the:equilibrium polarizability value, During rotational isomerization the minimum energy conformation corresponds to the maximum hardness and minimum polarizability values and the maximum energy conformation corresponds to the minimum hardness and maximum polarizability values, Density functional calculations confirm these observed trends, In general we have found that the conditions of maximum hardness and minimum polarizability complement the minimum energy criterion for molecular stability.