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Accurate cumulative reaction probabilities and densities of reactive states have been obtained for the reactions F+H2→H+HF and F+D2→D+DF using the hyperspherical (APH) coordinate scattering method developed by Pack and Parker. Results for scattering energies ranging from threshold to 0.7 eV above threshold have been obtained using two different potential energy surfaces that have been proposed for this system. The predicted peaks in the densities of reactive states are due to the presence of scattering resonances. The characteristic steps in the cumulative reaction probabilities are used to identify the resonance peaks due to quantized dynamical bottlenecks. The trapped-state or Feshbach-type resonances are identified by narrow oscillations in the density of reactive states. The positions of these resonance peaks in the density of reactive states are in good agreement with the peaks in the Franck–Condon factors for the photodetachment of H2F- and D2F-. Significant changes in resonance types are found for the T5a and 5SEC potential energy surfaces for H2F. While the assignment of resonance peaks presented is in reasonable agreement with the assignments suggested by Hahn and Taylor based on their classical mechanical analysis, there are some differences that will require further study. [ABSTRACT FROM AUTHOR]

The adiabatically adjusting principal-axis hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D) is used to obtain converged results for the reaction of helium with H+2 (v=1–4) for total angular momentum J=0. The ab initio potential energy surface computed by McLaughlin and Thompson and fitted by Joseph and Sathyamurthy is utilized for the HeH+2 interaction potential. The predicted energy dependence of the accurate 3D state-to-state reaction probabilities show clear evidence for quantum resonances. These resonances are even more numerous than those reported earlier for reduced dimensionality studies of this reaction. The calculated time delays for several of these resonances are found to be over 1 ps. Bending corrected rotating linear model (BCRLM) studies of this same reaction are also reported. These results provide useful insight in sorting out the nature and contribution of the resonances found in the 3D studies. [ABSTRACT FROM AUTHOR]

Accurate 3D coupled channel calculations for total angular momentum J=0 for the reaction F+H2→HF+H using a realistic potential energy surface are analyzed. The reactive scattering is formulated using the hyperspherical (APH) coordinates of Pack and Parker. The adiabatic basis functions are generated quite efficiently using the discrete variable representation method. Reaction probabilities for relative collision energies of up to 17.4 kcal/mol are presented. To aid in the interpretation of the resonances and quantum structure observed in the calculated reaction probabilities, we analyze the phases of the S matrix transition elements, Argand diagrams, time delays and eigenlifetimes of the collision lifetime matrix. Collinear (1D) and reduced dimensional 3D bending corrected rotating linear model (BCRLM) calculations are presented and compared with the accurate 3D calculations. [ABSTRACT FROM AUTHOR]

We employ a unified molecular dynamics (MD), quantum mechanical approach to simulate the behavior of large collections of atoms at finite temperature. The nuclei are moved according to classical mechanics while the forces are computed via quantum mechanical models. Two approaches have been used: (1) the extended Hückel method, which is an approximate molecular orbital approach and (2) density functional theory based on the local density approximation and plane wave pseudopotential formulation. We compute properties from the MD trajectories of up to 4 ps duration for samples containing up to 250 atoms of lithium, sodium, and potassium. These results are compared to both previous calculations and experimental results. [ABSTRACT FROM AUTHOR]

The validity of transition state theory is examined for the bimolecular reaction of Li+HF→LiF+H. Accurate three-dimensional quantum scattering theory calculations of the cumulative reaction probability are reported for energies ranging from threshold (0.255 eV) up to 0.600 eV and a total angular momentum J of 0. Transition state theory estimates of the effect of both the entrance and exit channels on the cumulative reaction probability are reported for the same energy range and J value. The transition state theory results are found to provide an accurate description of the smoothed energy dependence of the cumulative reaction probabilities with a maximum disagreement between the two calculations of about 25% arising at the highest energy considered of 0.6 eV. [ABSTRACT FROM AUTHOR]

The Darakjian–Hayes direct method for determining quantum lifetimes for three atoms scattering in three physical dimensions is used to determine accurate state-to-state time delays for the reaction of helium with H+2 for total angular momentum J=0. These results are compared with the time delays obtained by numerical differentiation of the S-matrix elements generated using the APH (adiabatically adjusting principal-axis hyperspherical) formulation of Pack and Parker. The direct method was found to be accurate and efficient for calculating the energy derivatives of the S matrix. The calculated eigenvalues of Smith’s collision lifetime matrix (eigen lifetimes) for this reaction predict numerous long-lived metastable states, many with lifetimes over 0.5 ps. The extent of the coupling of metastable states to specific scattering states provides an indication of the nature and magnitude of the time delays associated with particular state-to-state scattering processes. The direct method for calculating the energy derivatives of the S matrix is also found to be accurate and efficient for determining the energy derivative of the cumulative reaction probability. [ABSTRACT FROM AUTHOR]

The behavior of benzene at high temperatures and pressures is studied using nonequilibrium molecular dynamics. The interatomic forces were generated using linear-scaling tight-binding electronic structure theory on systems containing 128 and 576 molecules. The shock Hugoniot, calculated directly from the simulations without predetermining the equation of state, is compared with experiment. Piston velocities of 4 km/s or greater result in a pressure-induced polymerization. This transition is consistent with the bend in the experimental measurements of shock versus piston velocity. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents an erratum for an article on direct calculation of collisional properties that require energy derivatives.