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  • 11
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4788-4801 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Novel Monte Carlo simulation techniques are presented for efficient isobaric–isothermal simulations of branched chains and polymer networks with tri- and tetra-functional sites. Molecular rearrangements are performed by means of extended continuum configurational bias moves applied to single-path polymer portions. Volume fluctuations are performed via slab moves, which are extended in this work to effectively handle networks of arbitrary complexity. These methods are applied to determine the volumetric properties of linear and branched chains (with athermal and square-well interaction sites). Novel results are also presented for the compressibility of athermal and thermal polymer networks having a perfect, diamondlike connectivity. © 1996 American Institute of Physics.
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  • 12
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Physical Chemistry 50 (1999), S. 377-411 
    ISSN: 0066-426X
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract This review provides a discussion of recent techniques for simulation of phase equilibria of complex fluids. Monte Carlo methods are emphasized over molecular dynamics methods. We describe recent developments, such as the use of expanded-ensemble, tempering, or histogram reweighting techniques. Our discussion of such developments is aimed at a general audience and is intended to provide an overview of the main advantages and limitations of each particular technique. References are provided to allow interested readers to identify and trace back most recent applications of a particular simulation technique. We conclude with general guidelines regarding selection of suitable simulation methods for particular problems and systems of interest.
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  • 13
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 7957-7966 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Virtual Gibbs ensembles (VGEs), a class of pseudo-ensemble Monte Carlo methods, are specialized in this work to simulate phase equilibrium for systems for which conventional direct methods (such as two-box Gibbs ensembles and one-box interfacial ensembles) are inadequate. It is shown that by removing the mass/volume balance constraints of conventional Gibbs ensembles, the resulting VGEs can be used to effectively simulate systems wherein (a) the number of molecules or the composition of one of the phases is to be kept constant and (b) the initial choices for the volume and number of molecules would preclude direct methods to convergence to a stable two-phase state. Applications of VGEs are presented for the simulation of vapor–liquid, vapor–solid, and liquid–solid equilibrium in single component systems and multicomponent sorption equilibria of gases in polymer melts. VGE simulations of solid–fluid coexistence entail the combination of elements of both interfacial ensembles and Gibbs ensembles. © 2002 American Institute of Physics.
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  • 14
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 1290-1298 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo simulations have been performed to examine the volumetric behavior of model nonionic gel systems where the solvent is at subcritical and supercritical conditions. It is found that the degree of swelling correlates well with the density changes of the solvent: swelling decreases as the solvent becomes less dense. As the temperature is increased at constant pressure, gel swelling exhibits a monotonic decreasing trend whose onset approximately agrees with the solvent's boiling point for subcritical pressures and, for supercritical pressure, with the location of the lower critical solution temperature (LCST) of the solvent–uncrosslinked polymer system. Evidence of an incipient first-order volume phase transition has been detected for polymeric networks with long strand lengths in contact with a supercritical solvent. © 1999 American Institute of Physics.
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  • 15
    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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  • 16
    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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  • 17
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8761-8772 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Pseudoensembles are formulated to conduct Monte Carlo simulations of phase coexistence in multicomponent systems. It is shown that this approach can be applied to different types of ensembles. In contrast to the conventional Gibbs ensemble that performs flash-type calculations, pseudoensembles can also be designed to perform bubble-point and dew-point calculations. It is also shown that pseudoensemble simulations provide some connections between histogram reweighting techniques, Gibbs ensemble simulations, Gibbs–Duhem integrations, and thermodynamic integrations. In particular, pseudoensembles naturally lead to the formulation of Gibbs–Duhem integrations. The methods proposed here are validated by simulating vapor-liquid phase equilibrium of model binary mixtures. © 1998 American Institute of Physics.
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  • 18
    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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  • 19
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2703-2710 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is proposed for calculation of the chemical potential of macromolecules by computer simulation. Simulations are performed in an expanded ensemble whose states are defined by the length of a tagged molecule of variable size. A configurational-bias sampling and a preweighting scheme are introduced to facilitate transitions between such states. The usefulness of the method is illustrated by calculations of the chemical potential of hard chain molecules over a wide range of densities. The method proposed here is shown to offer significant advantages over other available methods for calculation of chemical potentials, particularly for long chain molecules at high densities. © 1995 American Institute of Physics.
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  • 20
    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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