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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 4000-4009 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work, we present a simple approach for devising order parameters (OPs) for atomic systems based on pattern recognition techniques. It exploits the fact that all crystalline substances are characterized by a unique "signature" cell (SC) which is constructed using a central atom and its nearest NSC neighbors in a given crystal. The algorithm measures the local degree of similarity between a SC and the system to be analyzed. The best fit of a SC to NSC atoms surrounding a given atom in the system is determined by maximizing a fictitious energy of binding among those atoms and the SC atoms. The fictitious potential energy is designed to give maximum attractive energy for maximum overlap. The maximum binding energy of interaction attained in this process is used as a measure of similarity between the crystal structure and the system (i.e., as an OP). The proposed method provides a unified and intuitive approach for constructing relevant OPs for a given system. We used these OPs to characterize the order of different phases in the Lennard-Jones system and in a model silicon system. It is shown that these novel OPs give a more complete description and a better understanding of the structural order in amorphous silicon than conventional OPs. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9905-9911 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Monte Carlo simulation study has been conducted to assess the ability of recently proposed force fields to predict orthobaric densities, second virial coefficients, and P-V-T data for short and long alkanes. A new, modified force field is proposed that provides good agreement with experimental phase equilibrium and second virial coefficient data over wide ranges of temperature and chain length. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2911-2923 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Pseudo-ensemble simulations and Gibbs–Duhem integrations are formulated within the framework of the expanded grand canonical ensemble. Pseudo-isobaric–isothermal simulations are proposed in which volume moves are replaced by fluctuations in the number of molecular segments. For large systems of dense athermal polymers, this pseudo-isobaric–isothermal method is shown to achieve mechanical equilibration faster than both conventional volume moves and the recently proposed slab volume moves. Pseudo-ensembles are also discussed for Gibbs ensemble simulations and canonical simulation (of the chemical potential). It is shown that coexistence curves for pure homopolymers and polymer mixtures can be traced by performing a numerical integration of the Gibbs–Duhem equation based on (expanded) grand canonical simulations. The validity of the methods is demonstrated by tracing the vapor–liquid coexistence curve of pure square-well chains and the liquid–liquid binodal curve of a blend of square-well chains. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9858-9868 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo simulations of mesogenic polymers have been performed for single and binary systems in the bulk and for pure components in constrained environments. An expanded ensemble formalism was employed to facilitate fluctuations of the number of particles in grand canonical, Gibbs, and pseudo-Gibbs ensemble simulations. Phase diagrams were generated for pure systems of athermal 8-mers and 16-mers with varying degrees of flexibility, for a binary mixture of rigid and semiflexible 16-mers (at constant pressure and variable flexibility), and for a bidisperse mixture of rigid chains. Simulations of the partitioning of rigid 12-mer chains between a bulk and a slit pore revealed that, for a certain range of slit gaps, the preferential coplanar orientation of molecules at the proximity of the walls induces the formation of a nematic phase within the pore. A similar induction of a nematic phase was observed for anisotropic molecules dispersed in a stretched, swollen gel. For the bulk systems, our simulation results were compared to predictions of various theories. © 1997 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 793-810 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of Monte Carlo simulations are presented for the equilibrium swelling of athermal (hard-core) polymeric gels. The networks investigated in this study are defect-free and exhibit strand lengths that range from small to moderate. It is shown that at equilibrium, increasing pressure and strand length leads to higher solvent holdups by the gel. Computer experiments of swelling of gels in binary solvent mixtures indicate that the gel absorbs preferentially the component having the smaller molecular excluded volume. A simple mean-field theory is also presented to describe the swelling of athermal networks. Agreement between theory and simulations is good. © 1997 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4391-4394 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel formalism is presented for simulation of polymers in expanded grand canonical and expanded Gibbs ensembles. Molecular creation and destruction attempts are replaced by transition attempts between states of a tagged chain of variable length. Results are presented for expanded grand canonical simulations of hard-core chain fluids in the bulk and in a slit pore and for expanded Gibbs ensemble simulations of vapor–liquid equilibria for square-well chains. © 1996 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5642-5652 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently developed formalism that synthesizes the description of statistical mechanical ensembles and the extrapolation of simulation data thereof is extended to encompass the formulation of semigrand canonical (SGC) type of ensembles and of ensembles suitable for confined systems and reactive equilibrium. These extensions provide a convenient framework to cast several ensembles previously used and also to formulate new variants. The advocated framework and its associated notation system are used to give general relationships for performing Monte Carlo simulations (acceptance rules) and implementing extrapolation and perturbation calculations. It is also shown how extrapolation methods can be systematically used to target equilibrium states whose specification does not allow the use of a direct simulation approach. © 2001 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5653-5661 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By employing the Massieu formalism presented in the preceding paper, Monte Carlo methods and extrapolation techniques are combined to simulate polymers with chain-length polydispersity. Semigrand canonical and grand canonical ensembles supplemented by identity exchange moves and expanded-ensemble moves have been tailored to simulate both thermodynamic properties and the structure of various coarse-grained model systems. It is demonstrated that the proposed methods can be used to simulate systems with arbitrary molecular weight distribution (e.g., to match experimental data), bulk-pore partitioning of polymer solutions, and liquid–liquid equilibrium of polydisperse polymer melts. © 2001 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8444-8456 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A unified framework is developed to describe extrapolations from simulations performed in either a single system box or two subsystems at equilibrium (i.e., Gibbs ensembles). It is shown that the Gibbs ensemble can be used in conjunction with histogram reweighting and pseudo-ensemble techniques in order to map out more effectively different kinds of phase diagrams, in particular for binary and ternary systems. These extrapolation schemes allow the use of different phase-equilibrium specifications, some of which could not be simulated by conventional approaches. Novel semi-open and osmotic Gibbs ensembles are also described as counterparts of single-phase open and osmotic ensembles, respectively. Applications of the proposed methods are presented to the simulation of pressure-composition diagrams, bubble lines, and isoenthalpic partitioning. © 2000 American Institute of Physics.
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  • 10
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A novel method has been developed to generate volume fluctuations as required in isobaric-isothermal Monte Carlo simulations of flexible (or semi-flexible) chain molecules. While conventional global volume moves entail symmetric changes of the simulation box size, in the new method proposed here, volume changes consist of one-dimensional fluctuations of one of two sections of the system. One of the two sections is a thin slab where all molecular rearrangements are restrained to occur. A deterministric method has been implemented to reposition the molecular segments contained within the slab after a volume change is proposed. The performance of the new method is examined by studying the evolution towards equilibrium of homopolymer hard-core systems upon application of a step increase in the pressure. The new method is shown to be an order of magnitude faster than conventional methods in attaining the equilibrium density of our test systems.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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