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  • 1
    Electronic Resource
    Electronic Resource
    Monte Carlo studies of the origins of the compensation effect in a catalytic reaction (1991)
    s.l. : American Chemical Society
    Langmuir 7 (1991), S. 2539-2543 
    Details
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/la00059a024
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  • 2
    Electronic Resource
    Electronic Resource
    Tracer-diffusion coefficients for both localized and nonlocalized adsorption: Theory and molecular-dynamics simulation (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 8694-8704 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a quantitative analysis of tracer diffusion in a molecular-dynamics simulation of the adsorption of an isolated ethane molecule on Pt(111). In particular, we examine the deviations between the tracer diffusion of ethane in the simulations and the assumptions of the nearest-neighbor adsorbate-hopping model at temperatures for which the kinetic energy of the molecule approaches and exceeds the diffusion-barrier energy. Our method of analysis can be implemented experimentally, with techniques such as scanning-tunneling microscopy. We show that the adsorbate-hopping model cannot accurately describe tracer diffusion at any of the temperatures probed. This is because ethane exhibits very long flights with flight times that are not negligible compared to the time required for the molecule to escape from a binding site. We propose a new formula for the diffusion coefficient that includes the influence of non-nearest-neighbor jumps with non-negligible flight times. In the limit of low temperatures, this expression reduces to a hopping model while, at high temperatures, our model predicts that the diffusivity becomes analogous to that for a two-dimensional gas. We show that our model quantitatively describes the tracer diffusion of ethane on Pt(111) in molecular-dynamics simulations over a wide temperature range, spanning both localized and nonlocalized adsorption. We comment on future research directions that may lead to a quantitative model of tracer diffusion in other similar systems. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470126
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  • 3
    Electronic Resource
    Electronic Resource
    A molecular-dynamics simulation study of the adsorption and diffusion dynamics of short n-alkanes on Pt(111) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 11021-11030 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using molecular-dynamics studies and static potential-energy minimization, we have resolved the mechanisms by which n-alkanes (ethane through n-decane) diffuse on a model Pt(111) surface in the low-coverage limit of a single adsorbed molecule. Our simulations reproduce all of the experimental trends seen for the adsorption and diffusion of C3–C6 on Pt(111) and Ru(001). The short alkanes (C2–C8) behave as rigid rods and their motion involves coupled translation and rotation in the surface plane. For this series, there is a linear increase of the diffusion barrier with the molecular chain length. We have analyzed the compliance of the motion of the assumptions of a nearest-neighbor hopping model. Although hopping motion can be observed for all of the molecules at sufficiently low temperatures, the hopping involves a significant fraction of long jumps. As the temperature increases, the adsorption becomes virtually delocalized. Despite the extensive deviations of the motion from the assumptions of a nearest-neighbor hopping model, the static diffusion-energy barriers, arising from the minimum-energy path for hops between nearest-neighbor binding sites, agree well with those obtained from the tracer-diffusion coefficients for butane, hexane, and octane. For these molecules, multiple-site hops and long flights appear to influence the values of the preexponential factors, which are too large. Neither the diffusion barrier nor the preexponential factor for ethane agrees well with theoretical estimates. We attribute these discrepancies to the smallness of the static diffusion barrier and/or the existence of unique dynamical behavior for this molecule. Due to the increased difficulty of in-plane rotation and increased mismatch between the geometries of the molecule and the surface, the diffusion barrier for n-decane drops below that for n-hexane. The characteristic mechanism of motion for n-decane involves significant C–C–C bond-angle bending. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467853
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  • 4
    Electronic Resource
    Electronic Resource
    Non-Arrhenius behavior in the initial rate of a catalytic-surface reaction: Theory and Monte Carlo simulation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 10028-10037 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have identified the factors contributing to the compensation effect in the initial rate of a bimolecular surface reaction, the kinetics of which are influenced by adsorbate lateral interactions. A simple theory, based on the quasichemical approximation, can predict the temperature ranges over which compensation is the most pronounced in Monte Carlo simulations of the initial rate. Both the simulations and the theory reveal an interesting phenomenon—apparent negative activation energies, which occur when the activation energy for reaction increases with increasing temperature faster than kBT. This phenomenon could contribute to experimentally observed decreases seen in the rate of the CO oxidation reaction on several single-crystal metal surfaces. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467991
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  • 5
    Electronic Resource
    Electronic Resource
    Theoretical foundations of dynamical Monte Carlo simulations (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 1090-1096 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo methods are utilized as computational tools in many areas of chemical physics. In this paper, we present the theoretical basis for a dynamical Monte Carlo method in terms of the theory of Poisson processes. We show that if: (1) a "dynamical hierarchy'' of transition probabilities is created which also satisfy the detailed-balance criterion; (2) time increments upon successful events are calculated appropriately; and (3) the effective independence of various events comprising the system can be achieved, then Monte Carlo methods may be utilized to simulate the Poisson process and both static and dynamic properties of model Hamiltonian systems may be obtained and interpreted consistently.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461138
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  • 6
    Electronic Resource
    Electronic Resource
    Effects of chain branching on the structure of interfacial films of decane isomers (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1653-1663 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This work is a molecular dynamics simulation study of the influence of chain branching on the structures of molecular films physically adsorbed on solid surfaces. The model systems considered are free-standing films of three C10H22 isomers: n-decane, i-decane (2-methylnonane), and t-butyl-hexane (2,2-methyloctane), on a Pt(111) surface. Proper potential models for the t-butyl group were developed and verified. All of these films exhibit layering and in-layer ordering near the surface and we have quantified these features. In general, layered molecules tend to lie with their molecular axes and backbone planes parallel to the surface. In-layer ordering is observed for molecules in layers near the surface. Neighboring molecules within ordered layers tend to align with their long molecular axes parallel to each other. n-Decane, a symmetric chain molecule, exhibits the strongest layering and in-layer ordering. The structures of i-decane films are very similar to those of n-decane films and show only slightly less order. t-Butyl-hexane films have a novel pillared layered structure, in which a few randomly distributed molecules orient themselves with the t-butyl end near the surface and the alkyl tail perpendicular to the surface. These molecules are surrounded by parallel, layered molecules. We discuss the implications of our findings for solvation forces in confined thin films of these molecules. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475536
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  • 7
    Electronic Resource
    Electronic Resource
    Smart Monte Carlo for accurate simulation of rare-event dynamics: Diffusion of adsorbed species on solid surfaces (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 686-695 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We introduce a dynamical Smart Monte Carlo algorithm and assess its applicability for simulating the rare-event dynamics of adsorbate diffusion. Using the dynamical Smart Monte Carlo method, we simulate the self-diffusion of an adatom in the Cu/Cu(001) and Rh/Rh(111) systems and we compare the simulated diffusion coefficients to values arising from molecular dynamics and transition-state theory. We find that the accuracy of Smart Monte Carlo is sensitive to details of the potential-energy surface. For Cu/Cu(001), the agreement between dynamical Smart Monte Carlo, molecular dynamics, and transition-state theory is excellent. A similar comparison for the Rh/Rh(111) systems shows discrepancies between these three techniques. We find that the origins of the discrepancies in the Rh/Rh(111) system are transition-state recrossings, for small simulation time steps, and low escape rates of the adatom from the binding sites, at large time steps. We examine the sampling and dynamics in trajectories using a smaller time step for motion perpendicular to the surface than that for parallel motion. These studies show that low Smart Monte Carlo escape rates in the Rh/Rh(111) system can be correlated to excessive sampling, beyond the configurational space of the potential-energy minimum, at large time steps. Recrossings can be understood to arise from the absence of velocity correlations in the low-friction, transition-state region and can be minimized through the use of a large time step for parallel motion. With the appropriate choice of simulation time steps it is possible to improve the agreement between dynamical Smart Monte Carlo and more rigorous dynamical techniques. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471895
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  • 8
    Electronic Resource
    Electronic Resource
    Diffusion mechanisms of short-chain alkanes on metal substrates: Unique molecular features (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1626-1635 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used transition-state theory to study the diffusion of short n-alkanes (n-butane–n-decane) on a model Pt(111) surface. We have determined diffusion energy barriers, preexponential factors, and characteristic mechanisms by which these molecules diffuse. Our studies reveal novel features associated with molecular mobility, including: Non-nearest-neighbor hops, local minima, conformational correlations, and directional anisotropy induced by molecular orientation. We examine factors that contribute to the relationship between molecular size and mobility. Tracer-diffusion coefficients for these molecules are best described by a heterogeneous lattice model. Diffusion coefficients calculated using this model agree well with those from experimental studies. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475533
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular diffusion on solid surfaces: A lattice-model study (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 587-593 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular diffusion on surfaces does not adhere to the basic assumptions of the adsorbate hopping model. Large molecules such as n-alkanes can bind at more than one site on surfaces. Their diffusion involves multiple hops to various nearest and non-nearest neighbor sites. In a recent study [J. S. Raut and K. A. Fichthorn, J. Chem. Phys. 108, 1626 (1998)], we proposed a simple heterogeneous lattice model to describe the behavior of these molecules on surfaces. In this work, we have carried out kinetic Monte Carlo simulations to verify the model and study the tracer and chemical diffusion of these molecules at different coverages and temperatures. Interestingly the tracer diffusion of a single molecule can be described by a solution of the lattice model obtained using the simplifying assumption of uncorrelated hopping out of different sites. The coverage dependence of tracer diffusion can also be described by a simple lattice model. We compare results from the kinetic Monte Carlo simulations to molecular-dynamics simulations and demonstrate that a lattice-based hopping model does account for all the relevant features of short chain diffusion on surfaces. The chemical-diffusion coefficient increases with increasing coverage, due to a reduction in configurational entropy. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478115
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  • 10
    Electronic Resource
    Electronic Resource
    Comment on "Constant temperature molecular dynamics simulations by means of a stochastic collision model. II. The harmonic oscillator'' [J. Chem. Phys. 104, 3732 (1996)] (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1646-1647 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473974
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