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  • 1
    Electronic Resource
    Electronic Resource
    Effect of reactant rotation on hydrogen atom transfer (1990)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 112 (1990), S. 8165-8166 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00178a050
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  • 2
    Electronic Resource
    Electronic Resource
    A stationary phase study of mj rainbows in molecule-corrugated surface scattering (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6515-6522 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the stationary phase limit of the quantum mechanical full sudden S matrix for diatom-corrugated rigid surface scattering of Proctor, Kouri, and Gerber [J. Chem. Phys. 80, 3845 (1984)] we obtain expressions for the rotational actions j and mj of an inelastically scattered diatom. By integrating over all points on the lattice, we reduce these to functions of the polar angles θ and φ only. It is found that j is a strong function of θ, but almost independent of φ, whereas the reverse is true for mj. Both j(θ) and mj(φ) are continuous plots which exhibit extrema known to produce rainbow behavior in inelastic gas-phase scattering theory. We propose that this implies the existence of rainbows in the mj distribution, and show the dependence of these rainbows on various potential parameters, including corrugation, potential repulsion parameter, and lattice constant. The results explain earlier trajectory studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455371
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  • 3
    Electronic Resource
    Electronic Resource
    The effect of reagent rotation on chemical reactivity: F+H2 revisited (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6906-6918 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory calculations on the gas phase reaction F+H2 ( j)→HF+H have been carried out. Different reactivity trends were seen depending on whether there was a chemically significant and anisotropic well in the entrance channel of the potential surface. For those in which there is no such well, rotation may decrease reactivity at low values of j, but increases it thereafter. The reaction cross section SR ( j) decreases slowly from j=0, reaching a minimum near j=6 then increases again. This behavior has been reported for several systems, including H+H2, and seems to be the "canonical'' behavior for SR ( j) for most direct chemical reactions. For F+D2 the minimum does not occur until j=8. However, this does correspond to the same amount of rotational energy as the minimum for F+H2 . For potentials in which there is a deep anisotropic well, it is found that the j=0 results are dominated by the presence of the well, and that the SR ( j=0) is anomalously high. On such surfaces there is normally a sudden drop in cross section from j=0 to j=1, followed by an increase. The experimental findings of Lee's group [J. Chem. Phys. 82, 3045 (1985)] that the cross section increases on going from j=0 to j=1 probably precludes the possibility of a chemically significant well in the entrance valley. The rotational product state distribution for both types of potential is dominated by kinematics away from threshold, and does not show the same trends as the reaction cross section. The mean product vibrational quantum number 〈v'〉 can decrease at low j, then increase at higher j. This occurs only at collision energies close to threshold, and on potentials which have a tight bend force constant at the transition state. The more general case for this reaction is that 〈v'〉 increases with j. For the case in which the potential has no well the differential cross section shows precisely the same trends as does SR( j). All these trends can be explained using a simple model we have recently proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457359
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  • 4
    Electronic Resource
    Electronic Resource
    The effect of reagent rotation in the reaction OH( j)+H2( j')→H2O+H (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7424-7433 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory calculations on the reaction OH( j)+H2( j')→H2O+H have been carried out with j≤40 and j'≤15 on both the Schatz–Elgersma [Chem. Phys. Lett. 73, 21 (1980)] and the Rashed–Brown [J. Chem. Phys. 82, 5506 (1985)] potential energy surfaces. When there is no rotation in the OH, then a plot of reaction cross section, SR( j') resembles that for an atom–diatom system: Just above threshold, rotation decreases reactivity for small j', but increases it for high j'; at higher translational energies this trend is less obvious, but still present. When j'=0, then SR( j) is a complicated function, decreasing for low j, then climbing to a maximum, finally decreasing once more at very large values of j. We have also carried out calculations with isotopically substituted H in OH, and show that these effects scale as the mass of the hydrogen isotope. We show that this behavior is due to artifacts in both the potential surfaces. Using a simple model we are able to rationalize this behavior. Using this same model, together with a potential in which the artifacts are absent, we predict that rotation in OH will decrease reactivity rather less than was reported in previous trajectory studies [Schatz, J. Chem. Phys. 74, 1133 (1981); Rashed and Brown, J. Chem. Phys. 82, 5506 (1985)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454306
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  • 5
    Electronic Resource
    Electronic Resource
    The effect of OH rotation on the reaction OH(v=0, j)+H2→H2O+H (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3698-3699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction cross section for OH(v=0, j≤31)+H2→H2O+H has been calculated using quasiclassical trajectories at E=0.15, 0.20, and 0.30 eV. There is an initial increase in reactivity with j, probably due to long-range R→T transfer. At very high j, the cross section decreases, probably because of disruption of the transition state by the OH rotation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452968
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  • 6
    Electronic Resource
    Electronic Resource
    Optimal annealing schedules for two-, three-, and four-level systems using a genetic algorithm approach (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7964-7978 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An annealing schedule, T(t), is the temperature as function of time whose goal is to bring a system from some initial low-order state to a final high-order state. We use the probability in the lowest energy level as the order parameter, so that an ideally annealed system would have all its population in its ground-state. We consider a model system comprised of discrete energy levels separated by activation barriers. We have carried out annealing calculations on this system for a range of system parameters. In particular, we considered the schedule as a function of the energy level spacing, of the height of the activation barriers, and, in some cases, as a function of degeneracies of the levels. For a given set of physical parameters, and maximum available time, tm, we were able to obtain the optimal schedule by using a genetic algorithm (GA) approach. For the two-level system, analytic solutions are available, and were compared with the GA-optimized results. The agreement was essentially exact. We were able to identify systematic behaviors of the schedules and trends in final probabilities as a function of parameters. We have also carried out Metropolis Monte Carlo (MMC) calculations on simple potential energy functions using the optimal schedules available from the model calculations. Agreement between the model and MMC calculations was excellent. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481397
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular beam scattering measurements of differential cross sections for D+H2(v=0)→HD+H at Ec.m.=1.5 eV (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 6396-6419 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction D+H2→HD+H has been studied in a crossed molecular beam scattering experiment at a most probable collision energy of Ec.m.=1.5 eV. Angular and time-of-flight distributions of the HD product have been measured over a wide range of angles. The experimental data are compared with quasiclassical trajectory (QCT) calculations on the LSTH potential hypersurface. The QCT calculations fit the experimental data quite satisfactorily. The sensitivity of the experiment to the different properties of the cross section has been investigated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451470
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  • 8
    Electronic Resource
    Electronic Resource
    Quantum effects in Ar+HF rotationally inelastic scattering: A semiclassical interpretation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 304-314 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Invoking a semiclassical approach to state-to-state rotationally inelastic scattering, this study seeks to determine the origin of several novel features observed in earlier quantum mechanical calculations [Rawluk et al., Chem. Phys. Lett. 202, 291 (1993)]. These features were absent from comparable classical trajectory calculations. The semiclassical (classical path) method used here treats the relative motion of Ar and HF classically, while the HF rotation is treated quantum mechanically by expansion in a rigid rotor basis set. This semiclassical approach reproduces the exact quantum results very well. The time dependence of the classical path allows a detailed study of the role played by the potential energy surface governing the dynamics. In particular, the behavior of the expansion coefficients 〈Yjm||Ψ(t)〉 in the complex plane is very revealing. From this analysis, it is clear that the quantum effects stem from a balance between the attractive and repulsive parts of the potential.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466999
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  • 9
    Electronic Resource
    Electronic Resource
    Global geometry optimization of atomic clusters using a modified genetic algorithm in space-fixed coordinates (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4700-4706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a recent paper, Gregurick, Alexander, and Hartke [S. K. Gregurick, M. H. Alexander, and B. Hartke, J. Chem. Phys. 104, 2684 (1996)] proposed a global geometry optimization technique using a modified Genetic Algorithm approach for clusters. They refer to their technique as a deterministic/stochastic genetic algorithm (DS-GA). In this technique, the stochastic part is a traditional GA, with the manipulations being carried out on binary-coded internal coordinates (atom–atom distances). The deterministic aspect of their method is the inclusion of a coarse gradient descent calculation on each geometry. This step avoids spending a large amount of computer time searching parts of the configuration space which correspond to high-energy geometries. Their tests of the technique show it is vastly more efficient than searches without this local minimization. They report geometries for clusters of up to n=29 Ar atoms, and find that their computer time scales as O(n4.5). In this work, we have recast the genetic algorithm optimization in space-fixed Cartesian coordinates, which scale much more favorably than internal coordinates for large clusters. We introduce genetic operators suited for real (base-10) variables. We find convergence for clusters up to n=55. Furthermore, our algorithm scales as O(n3.3). It is concluded that genetic algorithm optimization in nonseparable real variables is not only viable, but numerically superior to that in internal candidates for atomic cluster calculations. Furthermore, no special choice of variable need be made for different cluster types; real Cartesian variables are readily portable, and can be used for atomic and molecular clusters with no extra effort. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472311
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  • 10
    Electronic Resource
    Electronic Resource
    A study of genetic algorithm approaches to global geometry optimization of aromatic hydrocarbon microclusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2208-2218 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out potential energy minimization calculations on benzene, naphthalene, and anthracene clusters using model potential energy functions. The primary purpose was to examine several techniques which use concepts from the field of genetic algorithms (GA). In particular, we compared the "traditional GA" in which the variables of the problem are coded into binary and genetic operations performed on these, and recent methods which use real-valued variables. Our primary technique, the "space-fixed modified GA" (SFMGA), also uses a conjugate gradient descent on the geometries generated by the GA. Our results show the convergence to the global minimum is greatly improved by the use of the descent minimization. In fact, it appears unlikely that the traditional GA's are useful for any but the very simplest clusters. We have also compared the SFMGA with simulated annealing (SA) and Wales and Doye's recent basin-hopping (BH) technique. We find our method to be superior to SA, and comparable to BH. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475601
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