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In this work, we investigate the liquid–vapor phase transition of the restricted primitive model of ionic fluids. We show that at the low temperatures where the phase transition occurs, the system cannot be studied by conventional molecular simulation methods because convergence to equilibrium is slow. To accelerate convergence, we propose cluster Monte Carlo moves capable of moving more than one particle at a time. We then address the issue of charged particle transfers in grand canonical and Gibbs ensemble Monte Carlo simulations, for which we propose a biased particle insertion/destruction scheme capable of sampling short interparticle distances. We compute the chemical potential for the restricted primitive model as a function of temperature and density from grand canonical Monte Carlo simulations and the phase envelope from Gibbs Monte Carlo simulations. Our calculated phase coexistence curve is in agreement with recent results of Caillol obtained on the four-dimensional hypersphere and our own earlier Gibbs ensemble simulations with single-ion transfers, with the exception of the critical temperature, which is lower in the current calculations. Our best estimates for the critical parameters are T*c=0.053, ρ*c=0.025. We conclude with possible future applications of the biased techniques developed here for phase equilibrium calculations for ionic fluids. [ABSTRACT FROM AUTHOR]

We have studied the thermodynamically significant low-energy conformations of 200 random 24-residue model proteins on a square lattice with the Rosenbluth and Rosenbluth (1955) chain growth algorithm combined with multilink additions and Boltzmann weighting. We use a model proposed by Dill (1985) that represents the protein as a connected sequence of hydrophobic and hydrophilic beads on the lattice with nearest-neighbor interactions between the constituent beads. Two interaction sets were investigated—attraction between just the hydrophobic beads and attraction between hydrophobic residues and also between hydrophilic residues. The distribution of energies found with attraction only between hydrophobic beads is broad and consistent with the previous results of Lau and Dill (1989). However, the low-energy states are highly degenerate and noncompact. When attraction between hydrophilic beads is included with the attraction between hydrophobic beads, the energy distribution is sharp. Also, the low-energy configurations are reasonably nondegenerate and compact. This indicates that even with this simple model, important characteristics of the low-energy states of the model proteins are sensitive to the details of the interaction set used. [ABSTRACT FROM AUTHOR]

We have performed Monte Carlo simulations of folding transitions of model proteins on a simple cubic lattice with the Rosenbluth and Rosenbluth (1955) chain growth algorithm combined with Boltzmann weighting and multilink additions. We use a model proposed by Dill (1985) that represents the protein as a connected sequence of hydrophobic and hydrophilic beads on the lattice, with nearest-neighbor interactions between the constituent beads. The algorithm allows the calculation of equilibrium folding curves for arbitrary sequences of chain lengths of at least 48 in three dimensions. This is significantly longer than the maximum chain length for which equilibrium curves can be determined with the Metropolis (1953) based algorithms used in previous work. The thermal denaturation curve for the model is found to vary greatly with the sequence used, in some special cases approximating closely the sharpness observed for real proteins. These special sequences appear to be simple, periodic arrangements of beads. However, it seems that the unique or almost unique lowest energy states that exist for certain sequences of moderate length approaching the length of the simplest real proteins cannot be recovered in feasible simulation times with our particular model/algorithm combination. [ABSTRACT FROM AUTHOR]

The chemical potential of lattice polymers is calculated in two alternative ways: by the Widom insertion method using the Rosenbluth and Rosenbluth sampling technique and by the modified Widom method, based on the insertion of one segment to an existing polymer chain. In the first part of this paper we present a detailed derivation of the modified Widom technique for lattice systems. We then proceed to calculate the chemical potential for chains of up to 50-mers in monomeric and polymeric solvents. We observe marked odd-even effects on the chemical potential. The density dependence of the chemical potential is found to vary with chain length. For most temperatures and densities studied the chemical potential of chain molecules in a fixed environment becomes linear in chain length for molecules longer than 10–20 segments. The results are compared to the classical lattice theories, which are found to be best at high densities, as expected based on previous investigations. [ABSTRACT FROM AUTHOR]

Investigates the phase equilibrium the modified Buckingham exponential-6 potential variations. Determination of thermodynamic properties through the Hamiltonian scaling grand canonical Monte Carlo; Relevance of the sampled densities in multiple Hamiltonians; Calculation of the critical parameters.

Monte Carlo simulation and quasichemical theory are used to study the phase behavior of a lattice model for oil–water–amphiphile ternary systems. Several short amphiphiles with varying tail and head lengths are studied. Two- and three-phase coexistence regions are observed as well as the formation of microemulsions. In contrast to previous work on this model, quantitative phase diagrams are determined for both symmetric and asymmetric amphiphiles. The removal and regrowth of whole chains by configurational bias methods is used to help equilibration and sampling. Near quantitative agreement is found between quasichemical theory and our simulations except when one of the phases self-assembles or in the vicinity of a critical point. In these areas the quasichemical theory is still qualitatively correct. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have performed Monte Carlo simulations of the transition of polymers from an expanded to a collapsed state on a cubic lattice using a variation of the scanning technique of Meirovitch. The technique allows the study of highly compact polymer conformations that could not be studied with previously available methods. Our results indicate that the universal scaling function α3(1-T/θ)n1/2 vs (1-T/θ)n1/2 has a pronounced maximum before leveling off in the fully collapsed regime. Available experimental data for this function only cover the range up to and including the maximum. Good agreement is obtained between our calculations and experimental data for the polystyrene/cyclohexane system. [ABSTRACT FROM AUTHOR]

Reports on the phase coexistence properties of polarizable Stockmayer fluids of reduced permanent dipoles and reduced polarizabilities calculated using Monte Carlo simulations. Calculation of thermodynamic properties from the grand partition function; Prediction of phase coexistence curves at temperatures below the critical point.

Molecular dynamics simulations were performed to study systematically the rapid mixing of a polymer blend in solution with a miscible nonsolvent. In agreement with experiments, we observe that polymers self-assemble into complex nanoparticles, such as Janus and core-shell particles, when the good solvent is displaced by the poor solvent. The emerging structures can be predicted on the basis of the surface tensions between the polymers as well as between the polymers and the surrounding liquid. Furthermore, the size of the nanoparticles can be independently tuned through the mixing rate and the polymer concentration in the feed stream; meanwhile, the composition of the nanoparticles can be controlled by the polymer feed ratio. Our results demonstrate that this process is highly promising for the production of structured nanoparticles in a continuous and scalable way with independent and precise control over particle size, morphology, and composition. Such tailored nanoparticles are highly sought after in various scientific and industrial applications, and our theoretical findings provide important guidelines for designing appropriate experimental fabrication processes. [ABSTRACT FROM AUTHOR]

We report measurements of the vapour–liquid coexistence densities and surface tension of fully flexible Lennard–Jones chain molecules ranging in length from 4 to 60 beads. We demonstrate that the surface tension for all chain lengths collapses to a single master curve when plotted according to the universal parachor correlation. We find a universal parachor exponent 3.79 ± 0.05 for conditions close to the critical point, with a deviation observed for the longest chains far below the critical point. [ABSTRACT FROM AUTHOR]