Estela Blaisten-Barojas

Many-body potential and structure for rhodium clusters

The many-body potential for ferromagnetic and paramagnetic rhodium clusters proposed in this work has 11 parameters (14 for the paramagnetic case) that are fitted on the energy surface of Rh2 through Rh6 clusters calculated from first principles within the generalized gradient approximation (GGA) of density functional theory. Under this potential the most stable ferromagnetic and paramagnetic cluster structures are generated up to Rh58. Additionally, the growth under several symmetries is pursued up to N=400. The face-centered-cubic (fcc) growth path is the most stable at that cluster size regime. An effective measure of the cluster stiffness is calculated as a function of cluster size displaying a monotone increase towards the bulk value. The melting temperature is about constant up to clusters with 45 atoms, presenting a sharp increase towards the bulk value at larger sizes.

Energetics, structure, and charge distribution of reduced and oxidized n-pyrrole oligomers: A density functional approach

Polypyrrole is a conjugated polymer prototype of conducting polymers. The energetically preferred spatial conformation of n-pyrrole oligomers (n=1-24) in both the reduced and oxidized phases is obtained and analyzed in this paper within the hybrid density functional theory. Binding energies, gap energies, radius of gyration, end-to-end distance, and vibrational frequencies are reported as functions of oligomer length. Reduced n-pyrrole are bent chains for all sizes showing a dramatic departure from planarity. Vibrational spectra of n-pyrrole oligomers indicate the presence of two fairly size-insensitive frequency regions, which increase in intensity with increasing oligomer size. Several oxidation levels were analyzed for n-pyrrole through the distribution of the carbon-carbon bond orders and single/double bond lengths. It is shown that the oxidation level is directly related to the way positive charge localizes along the n-pyrrole oligomer chain. If charge/n<13, the oligomers are bent and charge is delocalized; if charge/n>/=13, the oligomers are planar and charge notoriously localizes in n/charge regions along the backbone. Calculations with electronegative dopants show that charge localizes in the neighborhood of the dopant. It is demonstrated that one localized state in the gap between the highest occupied and lowest-unoccupied states appears for every +2e in the oxidation level. The band structure of infinite reduced polypyrrole gives a band gap energy in excellent agreement with experiment. The evolution of the band gap and the charge-localized band as a function of polypyrrole oxidation level is reported.

Effects of three-body interactions on the structure of clusters

The structure of 54 and 147 atom clusters was studied using molecular dynamics at constant temperature and incorporating three-body interactions to the usual pairwise additive potentials among atoms. The effects of three-body interactions of the triple-dipole and exchange overlap type on the aggregation of clusters are: (1) to diminish the coordination number resulting in a global expansion of the clusters; (2) to increase internal disorder in such a way as to lower the crystallization temperature; (3) to favor certain features characteristic of random close packing of spheres at the expenses of destroying locally paired tetrahedra and octahedra, in the temperature range 0.25 ⩽ T ⩽ 0.4.

Framework-Type Determination for Zeolite Structures in the Inorganic Crystal Structure Database

In this work a structural characterization of zeolite crystals is performed by identifying the framework type to which each zeolite belongs. The framework type is assigned for 1433 zeolitedatabase entries in the FIZ/NIST Inorganic CrystalStructureDatabase (ICSD) populating 95 framework types. These entries correspond to both natural and synthetic zeolites. Each ICSD entry is based on published work containing crystallographic information of the zeolite crystalline structure and some physical and chemical data. Today, the Structure Commission of the International Zeolite Association recognizes crystalline materials as belonging to the “zeolite” family only if they possess one of the approved framework types by the organization. Such information is of fundamental importance for identifying zeolites, for reference, for zeolite standards, for supporting the discovery of new zeolites, and for crystalline substance selection based on application. Unfortunately, framework-type information is not contained in the ICSD records. The long term goal of this work is filling such gap. Although the ICSD contains an extensive collection of zeolites, inclusion of zeolites belonging to the 191 accepted framework types could substantially expand such collection. The structural determination was achieved via several structuralanalysis methods based on numerical-computer implementations.

Energetics and Vibrational Analysis of Methyl Salicylate Isomers

Energetics and vibrational analysis study of six isomers of methyl salicylate in their singlet ground state and first excited triple state is put forward in this work at the density functional theory level and large basis sets. The ketoB isomer is the lowest energy isomer, followed by its rotamer ketoA. For both ketoB and ketoA their enolized tautomers are found to be stable as well as their open forms that lack the internal hydrogen bond. The calculated vibrational spectra are in excellent agreement with IR experiments of methyl salicylate in the vapor phase. It is demonstrated that solvent effects have a weak influence on the stability of these isomers. The ionization reaction from ketoB to ketoA shows a high barrier of 0.67 eV ensuring that thermal and chemical equilibria yield systems containing mostly the ketoB isomer at normal conditions.

Strontium clusters: Many-body potential, energetics, and structural transitions

A many-body potential for strontium clusters is developed with parameters fitted to the energy surface of strontium clusters containing up to ten atoms calculated within the density functional theory in the generalized gradient approximation. Structure and energetics of the most stable cluster isomers with up to 63 atoms are obtained with genetic algorithms. Additionally, the high resolution mass spectrum of strontium clusters up to Sr96 at finite temperature is provided. Several thermodynamic properties are studied under the many-body potential as a function of temperature. It is found that stability patterns, indicating how stable a cluster size is with respect to its neighboring sizes, change significantly with temperature. This behavior is due to structural transitions of the strontium clusters that occur at finite temperatures. A comparison with the experimental mass abundance indicates that only the structures above 400 K were observed experimentally. Very prominent magic numbers are predicted at 34...

Role of three‐body interactions in trimer binding

A semiclassical perturbation expansion is developed for the ground state energy of three identical particles interacting via pair and three‐body potentials. The result is applied for the ground state energy of three neon atoms, where the non‐pair‐additive terms make a small contribution, and for three beryllium atoms where such terms make a large contribution. Models are constructed for the pair and three‐body potentials of beryllium, using the results of self‐consistent field and dispersion energy calculations. The beryllium dimer and trimer are self‐bound.

Non-additive interactions in liquid helium solvent effects

The Sinanoglu solvent effect method is here modified to include the exchange-overlap three-body SCF term in addition to the usual triple-dipole term. This is accomplished using an analytical fit which gives a better picture of short range non-additive interactions. The application of this modified model to the dimers (4He)2, (3He)2 and 3He-4He in a liquid 4He environment leads to the lowering of the effective-pair potential energy curve. As a consequence the new terms play a significant role, within the limitations of the method, in the calculation of a bound state of helium dimers immersed in a liquid helium medium.

Silicon carbide nanostructures: a tight binding approach.

A tight-binding model Hamiltonian is newly parametrized for silicon carbide based on fits to a database of energy points calculated within the density functional theory approach of the electronic energy surfaces of nanoclusters and the total energy of bulk 3C and 2H polytypes at different densities. This TB model includes s and p angular momentum symmetries with nonorthogonal atomic basis functions. With the aid of the new TB model, minima of silicon carbide cagelike clusters, nanotubes, ring-shaped ribbons, and nanowires are predicted. Energetics, structure, growth sequences, and stability patterns are reported for the nanoclusters and nanotubes. The band structure of SiC nanotubes and nanowires indicates that the band gap of the nanotubes ranges from 0.57 to 2.38 eV depending on the chirality, demonstrating that these nanotubes are semiconductors or insulators. One type of nanowire is metallic, another type is semiconductor, and the rest are insulators.

Molecular dynamics study of the depolymerization reaction in simple polymers

Abstract A molecular dynamics experiment was designed to follow the sequence of depolymerization reactions occurring when a polymer degrades into its constituent monomers. This simulation addresses internal changes that certain materials undergo at the moment of burning. Polymer fragments were thermally generated from the random scission of longer polymer chains containing 50–950 units. Subsequently, these thermal fragments depolymerized and coiled dramatically, forming incipient “agglomerates”. These agglomerates cooled while depolymerizing; the cooling mechanism remarkably inhibits the depolymerization reaction and eventually terminates the degradation process leading behind a sample of cold stable agglomerates. The size distribution of the polymer fragments is given as well as the IR spectrum of a typical sample at 2000 K.