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  • 1
    Electronic Resource
    Electronic Resource
    Melting and freezing of small argon clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4283-4295 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An investigation of melting, freezing, and coexistence phenomena is presented for small clusters using Ar7, Ar8, Ar13, and Ar14 as specific representative examples. Combining the results of molecular dynamics simulations, especially short-time kinetic energy averages and quenching, with accurate calculations of the local minima and transition states illuminates the relationship between the potential energy surface and dynamical processes. The results are consistent with a recent general defect theory of melting.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457788
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  • 2
    Electronic Resource
    Electronic Resource
    Exploring potential energy surfaces with transition state calculations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4308-4319 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Means are presented for using stationary points in two ways. One, for well-understood potentials, elucidates relations between the form of the surface and the dynamics that it supports, including the determination of the effective molecular symmetry group. The other, for potentials of uncertain quality, provides a test for unphysical characteristics and suggests how the surface might be improved if it is found to be unsatisfactory in some respect. Our approach involves comparison of transition state calculations using the slowest slide and Cerjan–Miller algorithms for two example systems: the Lennard-Jones Ar7 cluster and the Handy–Carter many-body-expansion potential for the ground state of formaldehyde.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457790
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum chaos of Ar3: Statistics of eigenvalues (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3470-3476 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The successive diagonalization–truncation method is applied to the calculation of the vibrational eigenvalues of the Ar trimer bound by pairwise Lennard-Jones potentials. The statistics of the eigenvalues reveal strongly chaotic behavior of the cluster, consistent with the classical dynamics studies. Moreover, the zero-point energy is higher than the highest energy at which regular dynamics were found classically, indicating that for all energies physically accessible to the cluster, the dynamics are chaotic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456876
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  • 4
    Electronic Resource
    Electronic Resource
    Vibrational predissociation in OH–Ar (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 5878-5879 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results are presented showing that the vibrational predissociation dynamics of OH−Ar differs greatly in its ground and excited electronic states. Measurements of the vibrational predissociation lifetime and the nascent internal state distribution of the OH product.(AIP)
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456395
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  • 5
    Electronic Resource
    Electronic Resource
    Melting and phase space transitions in small clusters: Spectral characteristics, dimensions, and K entropy (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1681-1694 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Grassberger–Procaccia method has been employed to study the transitions which occur as a classical Ar3 cluster, modeled by pairwise Lennard-Jones potentials, passes from a rigid, solid-like form to a nonrigid, liquid-like form with increasing energy. Power spectra and lower bounds on the fractal dimensions and K entropies are presented at several energies along the caloric curve for the Ar3 cluster. In addition, the full spectrum of Liapunov exponents has been computed at these same energies to get an accurate value of the K entropy. Chaotic behavior, though relatively small, is observed even at low energies where the power spectrum displays largely normal-mode structure. The degree of chaotic behavior increases with energy at energies where some degree of regularity is observed in the spectrum. However, at energies that just allow the system to pass into and across saddle regions separating local potential minima, the phase space appears to be separable into a region within the equilateral triangle potential well where the behavior is highly chaotic, and a region of lower dimensions and less chaos around the saddle of the linear configuration. Dimensions from approximately three to eight are observed. A clear separability of time scales for establishment of different extents of ergodicity permits the determination of fractal dimensions of the manifold on which the phase points moves, for time scales of physical, i.e., observable significance. We believe this to be the first evaluation of the dimensionality of the space on which the phase point moves, for a Hamiltonian system displaying this range of dimensions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455114
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  • 6
    Electronic Resource
    Electronic Resource
    An electron diffraction and ab initio investigation of cyclopropylisothiocyanate (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 4313-4318 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular structure of cyclopropylisothiocyanate, c-C3H5NCS, has been investigated at 35 °C by gas phase electron diffraction. Two distinct conformers, trans and cis, were identified with the isomeric proportion being 72±5% trans at this temperature. The structural parameters were determined by combining the electron diffraction data with the previously published rotational constants. A quadratic force field was used to calculate vibrational corrections. The important distances (r@B|av,A(ring)) and angles ((angle)*av, deg) with their uncertainties in parentheses were determined to be for the trans conformer: r(C1–C2)=1.520(3), r(C2–C3) =1.515(3), r(N=C)=1.193(3), r(C=S)=1.574(3), r(C1–N)=1.387(5), r(C–H) =1.072(5), (angle)C1NC=149.1(15), (angle)C2C1N=118.9(6), (angle)HCH=115.0(fixed), (angle)C2C1H=117.35(fixed), and (angle)NCS=177.7(20), whereas those that differed for the cis conformer were found to be: r(C1–N)=1.413(5), (angle)C1NC=150.8(17), and (angle)C2C1N=116.2(15). The rms amplitudes of vibration with their associated uncertainties have been determined for some of the distances. In addition, the geometry has been calculated by ab initio Hartree–Fock gradient calculations with geometry optimization at the STO-3G level and it is found that the computed structure is in reasonable agreement to the one derived from the electron diffraction experiment. Features of the structure are compared with those of similar molecules and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452747
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  • 7
    Electronic Resource
    Electronic Resource
    Melting and freezing in isothermal Ar13 clusters (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6456-6464 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microcanonical simulations have shown that Ar13 clusters have sharp but unequal melting and freezing energies. Between these energies, a hot solid-like form and a cooler, liquid-like form coexist in dynamic equilibrium. Monte Carlo and isothermal molecular dynamics simulations confirm that this coexistence behavior persists under canonical conditions as well. Many properties demonstrate the solid and liquid character of the two coexisting "phases.'' One previous result seemed to contradict this: Quirke and Sheng evaluated nearest neighbor angular distribution function P(θ); its nonzero value for θ=π/2 at 33 K was interpreted as that of a hot solid in a "premelting expansion.'' Actually, that result is the average of a bimodal distribution, one mode for the solid and the other for the liquid. The average shifts smoothly with T, and each form's P(θ) changes slightly with temperature. The solid has tiny nonzero probability for π/2. The liquid has a minimum probability there, but far above zero. Mean-square displacements and power spectra calculated at 33 K from the Nosé constant temperature molecular dynamics method exhibit properties which are clearly distinguishable and identifiable with two distinct phases, as they are under isoergic conditions. Hence our results can be added to the evidence supporting the picture for finite systems of two phases coexisting over a finite temperature and energy range.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452436
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  • 8
    Electronic Resource
    Electronic Resource
    How the range of pair interactions governs features of multidimensional potentials (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8745-8756 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By using the pairwise Morse potential as the principle vehicle we have explored the influence of the range of the pair potential on the structure of multidimensional potential-energy surfaces. A scaling relation and a set of reduced units are presented for the homoatomic pairwise-additive Morse potential. The reduced potential has one free parameter, βr0 or ρ0, regulating the range of interaction. Based on several diatomic species, the chemically important range of ρ0 is approximately 2≤ρ0≤7. The number of geometrically distinct minima and transition states depends on ρ0; the larger is ρ0, the narrower is the potential and the greater is the number of geometrically different minima. To illustrate this we found all minima and important low-energy transition states for the Morse six-and seven-particle clusters as functions of ρ0 in its chemically interesting range. From these the dominant mechanisms of isomerization of six- and seven-particle clusters are inferred and compared with experimental and theoretical results for main-group and transition-metal clusters. A nomenclature for saddle points and isomerizations is introduced. The saddle regions of the potentials reveal the dominance of diamond–square–diamond and edge-bridging mechanisms. Knowledge of the stationary points and rearrangement mechanisms allows us to determine the proper molecular symmetry groups and the topologies of the potential-energy surfaces at any arbitrary energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459263
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  • 9
    Electronic Resource
    Electronic Resource
    van der Waals vibrational dependence in the vibrational predissociation dynamics of OH–Ar (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6469-6479 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The OH–Ar vibrational predissociation lifetimes and OH product rotational state distributions are shown to change with van der Waals (vdW) state selection within the manifold of OH–Ar vibrational states correlating with OH A2Σ+(v'=1)+Ar(1S0). Excitations to pure vdW stretching levels result in similar product state distributions, but predissociation lifetimes that vary from 30±8 ps at v'vdW=0 to greater than 150 ps at vvdW=6. Excitations to assigned vdW bend-stretch combination bands result in product state distributions which differ from those observed after excitation of the pure vdW stretch and those differences are attributed to the form of the bending wave function. Rotational constants and band positions for OH–Ar features in the OH A2Σ+–X2Π3/2 0–0, 1–0, 1–1, 2–1, 1–2, and 2–2 regions are also presented. The spectroscopic analysis reveals details about the radial portion of the intermolecular potential between Ar (1S0) and hydroxyl radicals in the ground X2Π3/2 and excited A2Σ+ states. Vibrational excitation of the OH moiety induces measurable perturbations in the interaction potentials along the OH–Ar vdW stretching coordinate for both electronic states. These changes are reflected in the vibrational predissociation rates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458327
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  • 10
    Electronic Resource
    Electronic Resource
    Why separable vibrational modes dominate spectra (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2071-2072 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456000
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