Atom-exchange method

The atom-exchange method was developed by Tsai, Abraham, and Pound to speed barrier crossing in binary (two types of atoms) alloy cluster simulations. During the Metropolis walk two different types of atoms are periodically chosen, and their positions are exchanged. The exchange is accepted or rejected by the standard Metropolis acceptance probability. The utility of this method is naturally limited to systems of this particular type, namely, binary atomic clusters and liquids. 

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The rapid fission of a mass of or another heavy nucleus is the principle of the atomic bomb, the energy liberated being the destructive power. For useful energy the reaction has to be moderated this is done in a reactor where moderators such as water, heavy water, graphite, beryllium, etc., reduce the number of neutrons and slow those present to the most useful energies. The heat produced in a reactor is removed by normal heat-exchange methods. The neutrons in a reactor may be used for the formation of new isotopes, e.g. the transuranic elements, further fissile materials ( °Pu from or of the... 

Mercury was determined after suitable digestion by the cold vapour atomic absorption method [40]. Lead was determined after digestion by a stable isotope dilution technique [41-43]. Copper, lead, cadmium, nickel, and cobalt were determined by differential pulse polarography following concentration by Chelex 100 ion-exchange resin [44,45], and also by the Freon TF extraction technique [46]. Manganese was determined by flameless atomic absorption spectrometry (FAA). 

Koide et al. [537] have described a graphite furnace atomic absorption method for the determination of rhenium at picomolar levels in seawater and parts-per-billion levels in marine sediments, based upon the isolation of heptavalent rhenium species upon anion exchange resins. All steps are followed with 186-rhenium as a yield tracer. A crucial part of the procedure is the separation of rhenium from molybdenum, which significantly interferes with the graphite furnace detection when the Mo Re ratio is 2 or greater. The separation is accomplished through an extraction of tetraphenylarsonium perrhenate into chloroform, in which the molybdenum remains in the aqueous phase. 

While in principle all of the methods discussed here are Hartree-Fock, that name is commonly reserved for specific techniques that are based on quantum-chemical approaches and involve a finite cluster of atoms. Typically one uses a standard technique such as GAUSSIAN-82 (Binkley et al., 1982). In its simplest form GAUSSIAN-82 utilizes single Slater determinants. A basis set of LCAO-MOs is used, which for computational purposes is expanded in Gaussian orbitals about each atom. Exchange and Coulomb integrals are treated exactly. In practice the quality of the atomic basis sets may be varied, in some cases even including d-type orbitals. Core states are included explicitly in these calculations. 

Phytate analysis of muffins, diet composites and stools was performed by a modification of the ferric ion precipitation method for HS-I (5) and by an ion exchange method for HS-II and -III (6). Calcium was determined by atomic absorption spectrophotometry. 

The oxygen atoms in the silicalite framework can be classified into approximately three kinds by the isotope exchange method proposed by Endoh et al. [12]. On the exchange reaction of in the zeolite framework with C 02 in the gas phase at 773 K, the most reactive oxygen atoms were in the terminal SiOH... 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

The NEB method has been applied successfiilly to a wide range of problems, for example studies of diffusion processes at metal smfaces, multiple atom exchange processes observed in sputter deposition simulations, dissociative adsorption of a molecule on a smface, diffusion of rigid water molecules on an ice Di siuface, contact formation between metal tip and a smface, cross-slip of screw dislocations in a metal (a simulation requiring over 100,000 atoms in the system, and a total of over 2,000,000 atoms in the MEP calculation), g d diffusion processes at and near semiconductor smfaces (using a plane wave based Density Fimctional Theory method to calculate the atomic forces). In the last two applications the calculation was carried out on a cluster of workstations with the force on each image calculated on a separate node. 

All known isotopes of einsteinium are radioactive. The first evidence of lltcir existence was obtained by ion-exchange methods applied lo coral rocks obtained from Eniwetok Atoll alter (he thermonuclear explosion. The first pure isotope found was 3,3Es produced by prolonged treatment of plutonium-239 with neutrons in the Area. Idaho. Testing Reactur. The most stable is JMEs. half-life 270 days, and therefore the mass number 254 is carried in the atomic weight tabic. Others include - Es- Es. 48Es- Es, and s Es. Es. 

Adding immensely to the complexity of the experiment was the absolute necessity for the chemical separation of the one atom of element 101 from the 109 atoms of einsteinium in the target and its ultimate, complete chemical identification by separation with the ion exchange method. This separation and identification would presumably have to take place in a period of hours, or perhaps even one horn or less, since the half-life of new nuclide was unknown. Furthermore, the target material had a 20-d half-life, and one needed a nondestructive technique allowing reuse of the target material. 

Many-body calculations which go beyond the Hartree-Fock model can be performed in two ways, i.e. using either a variational or a perturbational procedure. There are a number of variational methods which account for correlation effects superposition-of-configurations (or configuration interaction (Cl)), random phase approximation with exchange, method of incomplete separation of variables, multi-configuration Hartree-Fock (MCHF) approach, etc. However, to date only Cl and MCHF methods and some simple versions of perturbation theory are practically exploited for theoretical studies of many-electron atoms and ions. 

Grabinski [12] has described an ion exchange method for the complete separation of the above four arsenic species, on a single column containing both cation and anion exchange resins. Flameless atomic absorption spectrometry with a deuterium arc background correction is used as a detection system for this procedure. This detection system was chosen because of its linear response and lack of specificity for these compounds combined with its resistance to matrix bias in this type of analysis. 

Aggett and Kadwani [13] employed a two stage single column anion exchange method using sodium hydrogen carbonate and chloride as eluate anions. These species appear to have no adverse effects in subsequent analytical procedures. Its successful application is dependent on careful control of pH. Analyses were performed by hydride generation atomic absorption spectroscopy... 

The VBSSA rule can be used to select relevant VB structures and construct the semiquantitative and quantitative curve-crossing VB diagram. Combining the MO method and VB calculations, we investigated atom exchange reactions H + HLi —> H2 + Li and H + LiH—>HLi + H, and discussed the effect of energy differences between the VB structures on the activation energy and properties of the transition state [70],... 

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