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  • 1
    Electronic Resource
    Electronic Resource
    Phase changes in 38-atom Lennard-Jones clusters. II. A parallel tempering study of equilibrium and dynamic properties in the molecular dynamics and microcanonical ensembles (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 10350-10357 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the 38-atom Lennard-Jones cluster with parallel tempering Monte Carlo methods in the microcanonical and molecular dynamics ensembles. A new Monte Carlo algorithm is presented that samples rigorously the molecular dynamics ensemble for a system at constant total energy, linear and angular momenta. By combining the parallel tempering technique with molecular dynamics methods, we develop a hybrid method to overcome quasiergodicity and to extract both equilibrium and dynamical properties from Monte Carlo and molecular dynamics simulations. Several thermodynamic, structural, and dynamical properties are investigated for LJ38, including the caloric curve, the diffusion constant and the largest Lyapunov exponent. The importance of insuring ergodicity in molecular dynamics simulations is illustrated by comparing the results of ergodic simulations with earlier molecular dynamics simulations. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481672
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  • 2
    Electronic Resource
    Electronic Resource
    Collapse of Lennard-Jones homopolymers: Size effects and energy landscapes (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2642-2649 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collapse of Lennard-Jones homopolymers is investigated by means of Monte Carlo simulations and the inherent structure/superposition approach, with special emphasis on finite size effects. At thermal equilibrium, the polymers undergo a series of phase changes from the zero temperature folded state to a coexistence state, a molten globule state, the coil state, and finally to a high-temperature "soft" state where the bond lengths vary significantly from their equilibrium value. The correlation between the thermodynamic characteristics of the polymers and those of clusters is interpreted in terms of the energy landscapes of the two systems. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1435572
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum partition functions from classical distributions: Application to rare-gas clusters (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 7312-7329 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the thermodynamic behavior of quantum many-body systems using several methods based on classical calculations. These approaches are compared for the melting of Lennard-Jones (LJ) clusters, where path-integral Monte Carlo (PIMC) results are also available. First, we examine two quasiclassical approaches where the classical potential is replaced by effective potentials accounting for quantum corrections of low order in (h-dash-bar). Of the Wigner–Kirkwood and Feynman–Hibbs effective potentials, only the latter is found to be in quantitative agreement with quantum simulations. However, both potentials fail to describe even qualitatively the low-temperature regime, where quantum effects are strong. Our second approach is based on the harmonic superposition approximation, but with explicit quantum oscillators. In its basic form, this approach is in good qualitative agreement with PIMC results, and becomes more accurate at low temperatures. By including anharmonic corrections in the form of temperature-dependent frequency shifts, the agreement between the quantum superposition and the PIMC results becomes quantitative for the caloric curve of neon clusters. The superposition method is then applied to larger clusters to study the influence of quantum delocalization on the melting and premelting of LJ19, LJ31, LJ38, and LJ55. The quantum character strongly affects the thermodynamics via changes in the ground state structure due to increasing zero-point energies. Finally, we focus on the lowest temperature range, and we estimate the Debye temperatures of argon clusters and their size variation. A strong sensitivity to the cluster structure is found, especially when many surface atoms reorganize as in the anti-Mackay/Mackay transition. In the large size regime, the Debye temperature smoothly rises to its bulk limit, but still depends slightly on the growth sequence considered. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1359768
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  • 4
    Electronic Resource
    Electronic Resource
    Characterization of anharmonicities on complex potential energy surfaces: Perturbation theory and simulation (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9627-9636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have systematically investigated the effect of anharmonicity on the equilibrium properties of systems with a complex potential energy surface. Anharmonicities are modeled by the temperature dependence of the harmonic frequencies {νi} near a stationary point of the PES. The low-temperature behavior is described by a simple thermal expansion ν(i)(β)=ν0(i)[1−α1(i)/β+α2(i)/2β2+(centered ellipsis)], where the coefficients {αj(i)} are obtained from perturbation theory. Using a simple diagrammatic representation, we give the complete expressions for the first two coefficients α1 and α2 in terms of derivatives of the potential. This approach is illustrated for the example of a bulk Lennard-Jones system of 32 particles, in both the solid and the liquid states. We also determine the anharmonic frequencies from reversible-scaling Monte Carlo simulations, which appear particularly well suited to this problem. As an example, we have studied a model biopolymer that exhibits significant first and second order anharmonicities. To show the importance of treating anharmonicities properly, we have calculated the caloric curve (heat capacity) of the quantum Ne13 cluster in both the classical and quantum regimes. For this calculation we have used a superposition approximation and exact anharmonic classical corrections to second order in perturbation theory. When every vibrational mode of each inherent structure is treated separately, we find good agreement between our results and previous quantum Monte Carlo calculations.© 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415462
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  • 5
    Electronic Resource
    Electronic Resource
    Mechanisms of phase transitions in sodium clusters: From molecular to bulk behavior (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2888-2908 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The thermodynamics of sodium clusters is investigated by means of a classical empirical potential and a simple quantal tight-binding model. Neutral and singly charged clusters of sizes ranging from 8 to 147 atoms are considered. A very particular attention is paid to the optimization and sampling problems. We determine the lowest-energy structures (global minima) with the "basin-hopping" technique, and the finite-temperature simulations are improved by using the "q-jumping" method and put together with the multiple histogram method. The clusters geometries may be very different on the model used, but also on the ionic charge, up to the size of about 40 atoms. The thermodynamical analysis is performed near the solid–liquid transition by calculating the complete calorific curves (heat capacities) as well as some microscopic parameters to probe the dynamics on the energy landscapes, including the spectra of isomers found by periodic quenching, isomerization indexes and the Lindemann parameter δ. Up to the largest sizes, we find that the heat capacity generally displays several features within the two models, although structural differences in the lowest-energy isomers usually induce different calorific curves. These premelting phenomena are characteristic of isomerizations taking place in a limited part of the configuration space. The thermodynamics appears to be directly related to the lowest-energy structure, and melting by steps is favored by the presence of defects on its surface. We estimate the melting temperatures Tmelt(n) and latent heats of melting L(n), and we observe two very different behaviors of their variations with the size n. Below about 75 atoms, both Tmelt and L exhibit strong non-monotonic variations typical of geometric size effects. This "microscopic" behavior is caused by the dominating premelting effects, and is replaced by a more "macroscopic" behavior for sizes larger than about 93 atoms. The premelting phenomena become there less important, and the melting process is much like the bulk solid–liquid phase transition rounded by size effects. The continuous variations displayed by the melting temperature are the only remains of cluster size effects. The models used are discussed and criticized on the basis of the similarities and discrepancies between their predictions and the experimental data. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480862
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  • 6
    Electronic Resource
    Electronic Resource
    On the convergence of global thermal properties of clusters extracted from simulations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1315-1317 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using standard classical molecular dynamics, we calculate the complete caloric curves of various metallic, ionic, or van der Waals clusters of different sizes. The apparent melting temperature is shown to be shifted to upper values as the simulation length τ is decreased. The shift ΔTmelt roughly scales as Nα/τβ, where N is the number of particles, α is about 0.6, and β is about 2.1. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481909
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  • 7
    Electronic Resource
    Electronic Resource
    Spectroscopy of calcium deposited on large argon clusters (2002)
    Springer
    The European physical journal 21 (2002), S. 153-161 
    Details
    ISSN: 1434-6079
    Keywords: PACS. 36.40.-c Atomic and molecular clusters – 82.33.Hk Reactions on clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Spectroscopic experiments have been performed, providing emission and excitation spectra of calcium atoms trapped on argon clusters of average size 2 000. The two experimental spectra fall in the vicinity of the calcium resonance line 1P 1 → 1S0 at 422.9 nm. The excitation spectrum consists in two bands located on each side of the resonance line of the free calcium. In addition, Monte Carlo calculations, coupled to Diatomics-In-Molecule potentials are employed to simulate the absorption spectrum of a single calcium atom in the environment of a large argon cluster of average size 300. The theoretical absorption spectrum confirms the existence of two bands, and shows that these bands are characteristic of a calcium atom located at the surface of the argon cluster and correspond to the excited 4p orbital of calcium either perpendicular or parallel to the cluster surface. The precise comparison between the shape of the absorption spectrum and that of the fluorescence excitation spectrum shows different intensity ratios. This could suggest the existence of a non adiabatic energy transfer that quenches partly the fluorescence of trapped calcium. Another explanation, although less likely, could be a substantial dependence of the calcium oscillator strength according to the alignment of the calcium excited orbital with respect to the cluster surface. The emission spectrum always shows a band in the red of the resonance line which is assigned to the emission of calcium remaining trapped on the cluster. When exciting the blue band of the excitation spectrum, the emission spectrum shows a second, weak, component that is assigned to calcium atoms ejected from the argon clusters, indicating a competition between ejection and solvation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1140/epjd/e2002-00201-5
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  • 8
    Electronic Resource
    Electronic Resource
    Evidence for dependence of the sticking cross-section of atoms on small and medium-size van der Waals clusters (2000)
    Springer
    The European physical journal 8 (2000), S. 265-272 
    Details
    ISSN: 1434-6079
    Keywords: PACS. 34.50.-s Scattering of atoms, molecules, and ions - 36.40.-c Atomic and molecular clusters - 64.60.Qb Nucleation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: This paper introduces a simple model describing the cluster growth in supersonic expansions. The predicted terminal mean cluster size is compared to the available data in the case of argon. The agreement between the model and the experimental results requires that the cross-section describing the sticking of an atom on a cluster of size N scales like with in the range 0.34-0.44, well below the predicted by the simplest geometrical scaling argument. We explain this unexpected result in two steps. First, using Monte Carlo simulations, we check that the potential between an atom and a cluster is accurately represented by the Gspann and Vollmar potential, even at finite temperature. Then, using Langevin's approximation, we show that the sticking cross-section scales like N 1/3 for small to moderate N values and switches to the geometric scaling N 2/3 for very large N values. The crossover between these two scalings occurs when for argon, but the mean exponent over the size range 1-104 is 0.46. This N scaling of the sticking cross-section should play an important role whenever condensation is important as it modifies the kinetics of the early stages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100530050035
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  • 9
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    Entropic tempering: A method for overcoming quasiergodicity in simulation (2000)
    American Physical Society
    Details
    Publication Date: 2000-12-20
    Print ISSN: 1063-651X
    Electronic ISSN: 1095-3787
    Topics: Physics
    Published by American Physical Society
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  • 10
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    Wetting-to-Nonwetting Transition in Metal-CoatedC60 (2003)
    American Physical Society
    Details
    Publication Date: 2003-02-21
    Print ISSN: 0031-9007
    Electronic ISSN: 1079-7114
    Topics: Physics
    Published by American Physical Society
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