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  • Articles  (65)
  • American Chemical Society (ACS)  (28)
  • American Institute of Physics (AIP)  (27)
  • MDPI
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  • 1
    Electronic Resource
    Electronic Resource
    Computational studies of submicron probing of polymer surfaces. I (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7072-7085 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The atomistic details of the interaction of an atomic force microscopic (AFM) probe with a polymer surface are examined by using the molecular-dynamics method. It is found that the perturbation of the AFM probe can produce a deformation of the local structure of the polymer surface. The dynamics study reveals how the structural changes evolve during the surface probing experiment, and whether the deformations result in permanent or reversible structural damage upon removing the probe. The effects of probe features (radius of curvature) and load force on the surface deformation(s) and image resolution are investigated in both a constant-force and constant-height AFM mode. Load forces between 10−8 and 10−11 N were determined to be an optimum working range for nondestructive AFM probing of polymer surfaces, and consequently for well-resolved surface image production.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462539
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  • 2
    Electronic Resource
    Electronic Resource
    Computer experiments on the internal dynamics of crystalline polyethylene: Mechanistic details of conformational disorder (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6875-6889 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The atomistic details of the internal dynamics of a polyethylene-like crystal are studied using molecular dynamics. Crystals with up to 6100 chain atoms have been studied for up to 30 ps. A microscopic description of the atomic motion has been examined and a link to available experimental data on the macroscopic and microscopic motion is provided. The results show that the onset of a significant population of rotational isomers is strongly altered by the intermolecular forces. Typical rates for the formation of isomers are 1010 to 1012 s−1 at 350 K (depending on the size of the simulated crystal, which changes the overall nature of the intermolecular forces) and increase exponentially with temperature. The large number of created defects causes a continuous decrease in the end-to-end distance. Specific defects, however, have extremely limited lifetime (i.e., those suggested by molecular mechanics calculations). These results suggest that at the temperatures where annealing or deformation of metastable crystals is possible, only randomly generated defects cause the macroscopically observed changes. The defects should move under the free enthalpy gradient set up within the crystal toward a more stable location. The activation energy required for motion which ultimately results in mass transport or lamellar thickening can be shown to be temperature and chain-length dependent. The highly uncorrelated behavior of the creation and annealing of defects reveals the underlying chaotic nature of the "transition'' from an ordered crystal to a conformationally disordered crystal (CONDIS crystal). In the simulated case, the transition to the conformationally disordered state occurs gradually, involving little or no cooperative motion. This continuous transition to the condis state was suggested earlier on the basis of experimental evidence and is expected to occur in many other polymers in addition to and at lower temperature than possible additional first-order transitions to the condis state. Thermodynamic and kinetic parameters of the simulations have been determined and compared to the available experimental data with good agreement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458921
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  • 3
    Electronic Resource
    Electronic Resource
    A neural network approach to the study of internal energy flow in molecular systems (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 293-306 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neural networks are used to develop a new technique for efficient analysis of data obtained from molecular-dynamics calculations and is applied to the study of mode energy flow in molecular systems. The methodology is based on teaching an appropriate neural network the relationship between phase-space points along a classical trajectory and mode energies for stretch, bend, and torsion vibrations. Results are discussed for reactive and nonreactive classical trajectories of hydrogen peroxide (H2O2) on a semiempirical potential-energy surface. The neural-network approach is shown to produce reasonably accurate values for the mode energies, with average errors between 1% and 12%, and is applicable to any region within the 24-dimensional phase space of H2O2. In addition, the generic knowledge learned by the neural network allows calculations to be made for other molecular systems. Results are discussed for a series of tetratomic molecules: H2X2, X=C, N, O, Si, S, or Se, and preliminary results are given for energy flow predictions in macromolecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463628
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  • 4
    Electronic Resource
    Electronic Resource
    Infrared laser-induced chaos and conformational disorder in a model polymer crystal: Melting vs ablation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6081-6091 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics-based computer simulations are presented for the interaction of one and two infrared (IR) laser beams with a model polymer surface. When a single laser beam system is studied over a wide range of intensities, only melting of the polymer, or melting followed by bond dissociation, is observed for up to 100 picoseconds. In contrast, the two-laser simulation results exhibit a marked difference in the energy absorption behavior of the irradiated polymer which, in turn, results in multiple bond dissociations. The results for the one- and two-laser cases studied can be divided into four different classes of physical behavior: (a) the polymer remains in the solid state; (b) the polymer crystal melts; (c) the polymer ablates, but with significant melting (charring); or (d) the polymer ablates with minimal melting. Damage to the model polymer crystal from absorption of energy from either one or two lasers occurs through a mechanism that involves the competition between the absorption of energy and internal energy redistribution. The rate of energy loss from the absorption site(s) relative to the rate of absorption of energy from the radiation field determines rather the polymer melts or ablates (low absorption rates lead to melting or no change and high rates lead to ablation). A sufficiently large rate of energy absorption is only obtainable through the use of two lasers. Two lasers also significantly decrease the total laser intensity required to cause polymer crystal melting. The differences between the one- and two-laser cases are studied by adapting novel signal/subspace techniques to analyze the dynamical changes in the mode spectrum of the polymer as it melts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459496
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  • 5
    Electronic Resource
    Electronic Resource
    Symplectic integrators for large scale molecular dynamics simulations: A comparison of several explicit methods (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4062-4072 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We test the suitability of a variety of explicit symplectic integrators for molecular dynamics calculations on Hamiltonian systems. These integrators are extremely simple algorithms with low memory requirements, and appear to be well suited for large scale simulations. We first apply all the methods to a simple test case using the ideas of Berendsen and van Gunsteren. We then use the integrators to generate long time trajectories of a 1000 unit polyethylene chain. Calculations are also performed with two popular but nonsymplectic integrators. The most efficient integrators of the set investigated are deduced. We also discuss certain variations on the basic symplectic integration technique.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467523
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  • 6
    Electronic Resource
    Electronic Resource
    The onset of instability in nanostructures: The role of nonlinear resonance (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6619-6622 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory methods are used to examine the vibrational dynamics of carbon nanotubes. The results clearly demonstrate an integral relationship between the diameter and length of a nanotube and its positional stability: tubes having diameters smaller than 0.7 nm undergo large-amplitude motion. The origin of this motion is due to strong coupling(s) between the longitudinal (vibration along the length) and a ring breathing mode (vibration about the axis of the cylinder). It is shown that the vibrational frequency of these modes follow a simple scaling law: ωc∝1/C, ωL∝1/L, where C is the contour length around the end of the tube and L is the length of the tube along its axis. This law should be applicable to any isotropic material with a cylindrical shape and provides an analytical equation for predicting mechanical stability: When the frequencies have small integer ratios with one another, in particular a 1:2 ratio, instability will occur on a short time scale (this phenomena represents a nonlinear resonance controlled by the geometry of the system). © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469378
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  • 7
    Electronic Resource
    Electronic Resource
    Unimolecular reaction dynamics of dimethylnitramine (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 6889-6897 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unimolecular reaction dynamics of dimethylnitramine are studied using classical trajectories on three model potential-energy surfaces. Elimination of NO2 is the predominant reaction. The focus of this study is on the simple, bond-rupture reaction to give NO2. Rate constants are calculated for two of the potentials at two different energies and individual trajectories are examined to determined the nature of the energy flow in the molecule prior to reaction and the product energy distribution. Energy flow into the nitro group increases significantly prior to reaction. Energy becomes trapped in the nitro group as the N–N bond begins to break and rapidly exchanges between the NO2 bending and stretching modes. One of the potentials allows the concerted molecular elimination of HONO. The concerted process which involves hydrogen migration and subsequent HONO elimination accounts for less than 5% of the reactive trajectories. However, the HONO elimination reaction can be enhanced by exciting high CH stretch overtones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454386
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  • 8
    Electronic Resource
    Electronic Resource
    Intramolecular dynamics of the overtone-induced isomerization of methyl isocyanide (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5809-5819 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Overtone-induced isomerization of methyl isocyanide to methyl cyanide is studied by using classical trajectories on several potential-energy surfaces. The several potential-energy surfaces are variations of a potential which we have developed based on available experimental and ab initio results. The trajectory results for simple potentials which neglect stretch–bend interactions show that overtone excitation of a CH stretch to the v=6 level at total energies (including overtone excitation energy) of 75, 125, and 150 kcal/mol does not enhance the rate of isomerization. However, at an initial total energy of 200 kcal/mol, the isomerization rate is enhanced by as much as a factor of 3 by selective excitation of a CH stretching overtone. However, the mode specificity is sensitive to the potential-energy surface. When a more realistic potential is used in which the bending force constants are attenuated as a function of the bond lengths or in which nondiagonal quadratic coupling terms are included, the dominant reaction is CH bond dissociation. The rate coefficient for the CH bond dissociation is an order of magnitude greater than the rate of isomerization at 200 kcal/mol. The initial energy flow out of an excited CH stretch is rapid (occurring on a times scale of less than 0.5 ps) and is primarily into the methyl bending modes. The energy that flows into the bending modes does not transfer out over the time period of 5.4 ps that the trajectories were followed. The methyl bending modes act as an energy "sink''.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453505
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  • 9
    Electronic Resource
    Electronic Resource
    The effect of resonances on collisional energy transfer (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 1012-1021 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of resonances on the collinear inelastic scattering of CO2 with He, Ne, and Ar has been studied. The initial conditions for the CO2 molecule were chosen by using Poincaré surfaces of section to identify trajectories which lie on resonant tori. Near-resonant and nonresonant trajectories have also been examined. The dynamics of the CO2 molecule undergoes a dramatic change as a rare gas atom approaches. Resonances are created and destroyed and, in some cases, a transition from quasiperiodic to chaotic motion occurs. The energy transfer is enhanced for trajectories involving resonant or near-resonant CO2 states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453334
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  • 10
    Electronic Resource
    Electronic Resource
    An internal coordinate quantum Monte Carlo method for calculating vibrational ground state energies and wave functions of large molecules: A quantum geometric statement function approach (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5494-5502 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An internal coordinate quantum Monte Carlo (ICQMC) method using importance sampling is illustrated for a 100 atom model polyethylene chain. Importance sampling with an internal coordinate guiding wave function yields smoother, more physically reasonable wave functions and lower ground state energies than Cartesian importance sampling, in good agreement with normal coordinate analysis results. A novel geometric statement function (GSF) method for economizing expressions involving first and second derivatives of stretch, bend, and torsion internal coordinates by up to 2 orders of magnitude allows QMC calculations to be performed even for large molecules in reasonable times on standard workstations. The ICQMC method with quantum GSF is eminently suitable for large molecules with complicated, strongly coupled potential energy surfaces such as polymer chains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472405
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