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    PublicationOpen Access
    Long time scale molecular dynamics subspace integration method applied to anharmonic crystals and glasses
    (American Institute of Physics (AIP) Publishing, 1993) Space B.; Rabitz H.; Department of Mathematics; Aşkar, Attila; Faculty Member; Department of Mathematics; College of Sciences; 178822
    A subspace dynamics method is presented to model long time dynamical events. The method involves determining a set of vectors that span the subspace of the long time dynamics. Specifically, the vectors correspond to real and imaginary low frequency normal modes of the condensed phase system. Most importantly, the normal mode derived vectors are only used to define the subspace of low frequency motions, and the actual time dependent dynamics is fully anhannonic. The resultant projected set of Newton's equations is numerically solved for the subspace motions. Displacements along the coordinates outside the subspace are then constrained during the integration of the equations of motion in the reduced dimensional space. The method is different from traditional constraint methods in that it can systematically deduce and remove both local and collective high frequency motions of the condensed phase system with no a priori assumptions. The technique is well suited to removing large numbers of degrees of freedom, while only keeping the very low frequency global motions. The method is applied to highly anhannonic Lennard-Jones crystal and glass systems. Even in these systems with no intramolecular degrees of freedom or obvious separation of time scales, the subspace dynamics provides a speed up of approximately a factor of 5 over traditional molecular dynamics through use of a larger integration time step. In the cases illustrated here a single set of subspace vectors was adequate over the full time interval, although this is not expected to be true for all systems.
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    Migration of single iridium atoms and tri-iridium clusters on MgO surfaces: aberration-corrected STEM imaging and ab initio calculations
    (American Chemical Society (ACS), 2015) Han, Chang Wan; Iddir, Hakim; Curtiss, Larry A.; Browning, Nigel D.; Gates, Bruce C.; Ortalan, Volkan; Department of Chemical and Biological Engineering; Uzun, Alper; Faculty Member; Department of Chemical and Biological Engineering; College of Engineering; 59917
    To address the challenge of fast, direct atomic-scale visualization of the migration of atoms and clusters on surfaces, we used aberration-corrected scanning transmission electron microscopy (STEM) with high scan speeds (as little as similar to 0.1 s per frame) to visualize the migration of (1) a heavy atom (Ir) on the surface of a support consisting of light atoms, MgO(100), and (2) an Ir-3 cluster on MgO(110). Sequential Z-contrast images elucidate the surface transport mechanisms. Density functional theory (DFT) calculations provided estimates of the migration energy barriers and binding energies of the iridium species to the surfaces. The results show how the combination of fast-scan STEM and DFT calculations allow visualization and fundamental understanding of surface migration phenomena pertaining to supported catalysts and other materials.
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    Phase-space reconstruction in Hamiltonian systems through multiple time series
    (Elsevier Science Bv, 1997) Department of Chemistry; Atay, Fatihcan; Yurtsever, İsmail Ersin; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; 253074
    A method which combines the various time series originating from a single source in order to reconstruct the phase-space dynamics is applied to a chaotic Hamiltonian system. It is shown that for a large energy range the variation of the maximum Lyapunov exponents can be reproduced more accurately than the results obtained from a single time series, for both clean and noisy signals. Especially in cases where the maximum exponent is close to zero, using multiple series can give better results, providing a more reliable way of detecting chaos.
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    The CH-3 Sigma+ anion: inelastic rate coefficients from collisions with he at interstellar conditions
    (American Chemical Society (ACS), 2023) de la Fuente, Jorge Alonso; Sanz-Sanz, Cristina; Gonzalez-Sanchez, Lola; Wester, Roland; Gianturco, Francesco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129
    We present accurate ab initio calculations on several properties of a gas-phase system of interest in the interstellar medium (ISM), where the title molecular anion has been often surmised but not yet confirmed by observations. The CH-3 sigma+ constitutes the smallest term in the series of longer anionic polyynes which have been observed in the ISM (e.g., C4H-and several others). Hence, its dynamical behavior in collision with He atoms, one of the most abundant atoms in that environment, can provide quantitative indicators on the changes which can occur in the rotational state population of the title anion when driven by this collision dynamics. We therefore report an accurate evaluation of the full potential energy surface (PES) which acts between the molecular anion in its ground vibrational state and the He atom. The relevant inelastic scattering cross sections and the corresponding inelastic rate coefficients are then computed within a quantum treatment of the collisions. We find that the fairly small values of the final inelastic rate coefficients indicate state-changing processes by collisions to be inefficient paths for modifying the rotational state populations of this anion and therefore to aid its possible observation from direct radiative emission in the microwave region.