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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 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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11999-12010 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Novel formulations of Gibbs-Duhem integrations are proposed to trace phase diagrams for multicomponent fluid mixtures, including generalized bubble-dew lines, whereby the composition of one of the phases is specified, but may not be constant along the integration path. The algorithms presented in this work constitute an extension of a recently proposed pseudo-ensemble framework. Different types of phase equilibrium calculations with open ensembles are cast into a unique formulation, which leads to a simple and flexible integration scheme for stepping along a coexistence line. This approach is applied to trace phase diagrams for systems described by realistic force fields, including hydrocarbon mixtures. A variant of a Gibbs ensemble that permits simulation of bubble-dew point pressures is also implemented. © 1999 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 113 (2000), S. 11382-11392 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel Monte Carlo technique, the rebridging configurational-bias (RCB) method, has been developed to rearrange inner sections of chain molecules having strong intramolecular interactions along the backbone. The ability of sampling inner chain segments is important for the simulation of systems with low concentration of chain ends, such as polymers and molecules with cyclic structures. In the RCB method, inner segments are removed and then regrown site-by-site in a configurational-bias fashion. However, a short preliminary simulation is carried out to calculate weighting functions based on histograms of the separation distance between pairs of sites along the chain; these functions are used to bias the trial positions of growing inner sites so as to promote efficient chain closure. A look-ahead search scheme is employed for the sampling of the last two sites to increase the overall acceptance rate of the reconstruction process. The validity and performance of the new algorithm were tested by studying linear alkane systems of various chain lengths. Fast conformational equilibration was observed, from the level of local bond orientations to the level of the chain end-to-end vector orientations. Cyclic alkanes containing from 8 to 100 carbon atoms were also studied by using the RCB method. Our results for the conformational properties of cyclooctane are generally consistent with previous molecular dynamics (MD) results and with experimental data. The vapor–liquid coexistence curve of cyclooctane was also mapped out by using the RCB method in conjunction with a histogram reweighting technique for the analysis of isothermal–isobaric simulations. © 2000 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 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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1288-1290 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of molecular simulations are presented for the entropy-driven collapse of a single chain in an athermal solvent. The mean square radius of gyration (〈Rg2〉) is calculated for a flexible chain of length N immersed in a hard-sphere medium having a specified solvent chemical potential. For a ratio of chain bead diameter to solvent diameter of 0.2 and packing fraction η=0.44, we find that 〈Rg2〉∝N2ν with ν∼0.36 for N≥100; such exponent is consistent with that predicted by renormalization group arguments for the poor solvent regime (ν=1/3). The effects of chain length and density on 〈Rg2〉 are consistent with the predictions of the Edwards–Muthukumar theory for a chain in a quenched, random medium. © 1997 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 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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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1946-1956 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel approach is presented for the development of equations of state for chain molecules. The basic assumptions of the approach are supported by results of computer simulations of the chemical potential of athermal chains. Our model establishes a bridge between some elements of Wertheim's thermodynamic perturbation theory and the generalized Flory theory. New equations of state are presented for freely jointed tangent hard-sphere chains which are shown to be more accurate than other existing equations. Extensions of our approach are also presented for branched polymers and blends of polymers. © 1995 American Institute of Physics.
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