Fernando A. Escobedo

On the simulation of vapor–liquid equilibria for alkanes

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.

Mesophase behaviour of polyhedral particles

Translational and orientational excluded-volume fields encoded in particles with anisotropic shapes can lead to purely entropy-driven assembly of morphologies with specific order and symmetry. To elucidate this complex correlation, we carried out detailed Monte Carlo simulations of six convex space-filling polyhedrons, namely, truncated octahedrons, rhombic dodecahedrons, hexagonal prisms, cubes, gyrobifastigiums and triangular prisms. Simulations predict the formation of various new liquid-crystalline and plastic-crystalline phases at intermediate volume fractions. By correlating these findings with particle anisotropy and rotational symmetry, simple guidelines for predicting phase behaviour of polyhedral particles are proposed: high rotational symmetry is in general conducive to mesophase formation, with low anisotropy favouring plastic-solid behaviour and intermediate anisotropy (or high uniaxial anisotropy) favouring liquid-crystalline behaviour. It is also found that dynamical disorder is crucial in defining mesophase behaviour, and that the apparent kinetic barrier for the liquid-mesophase transition is much lower for liquid crystals (orientational order) than for plastic solids (translational order).

EXPANDED GRAND CANONICAL AND GIBBS ENSEMBLE MONTE CARLO SIMULATION OF POLYMERS

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.

Extended continuum configurational bias Monte Carlo methods for simulation of flexible molecules

The continuum configurational bias (CCB) Monte Carlo method has been extended to perform elementary moves that involve the rearrangement of inner segments of flexible chains. When regrowing inner sites, the continuity with the rest of the chain is ensured by disregarding those configurations that would imply an unrealistic elongation of the bonds once the chain is reconstructed. The formalism presented here also allows the simulation of branched chains and crosslinked‐network structures. The Monte Carlo elementary moves proposed in this work are used in conjunction with an alternative method of preferential sampling in which the segments to be rearranged are chosen from a preselected region of space. The performance and capabilities of the new moves are compared to those of standard CCB and crank‐shaft algorithms for simulation of melts and solutions of hard‐sphere chains at high densities. Our results indicate that the methods presented here provide a fast relaxation of the bond orientation and the end‐t...

Monte Carlo simulation of the chemical potential of polymers in an expanded ensemble

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.

Simulation and prediction of vapour-liquid equilibria for chain molecules

Monte Carlo simulations of phase equilibria for Lennard-Jones chains of intermediate length are performed in the Gibbs ensemble using configurational bias sampling. Simulations of phase equilibria for square-well chains of up to 100 segments are performed using the NPT-μ method and newly proposed Monte Carlo moves. A two-reference-fluid equation of state is developed to describe the pressure-volume-temperature properties of square-well and Lennard-Jones chains. The phase envelopes predicted by such an equation are in good agreement with results of simulations. This equation is also shown to be superior to models derived from first-order thermodynamic perturbation theory (TPT1).

Transition path sampling and forward flux sampling. Applications to biological systems

The last decade has seen a rapid growth in the number of simulation methods and applications dealing with the sampling of transition pathways of rare nanoscale events. Such studies are crucial, for example, for understanding the mechanism and kinetics of conformational transitions and enzymatic events associated with the function of biomolecules. In this review, a broad account of transition path sampling approaches is provided, starting from the general concepts, progressing to the specific principles that underlie some of the most important methods, and eventually singling out the so-called forward flux sampling method for a more detailed description. This is done because forward flux sampling, despite its appealing simplicity and potential efficiency, has thus far received limited attention from practitioners. While path sampling methods have a widespread application to many types of rare transitional events, here only recent applications involving biomolecules are reviewed, including isomerization, protein folding, and enzyme catalysis.

NOVEL PSEUDOENSEMBLES FOR SIMULATION OF MULTICOMPONENT PHASE EQUILIBRIA

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.

MONTE CARLO SIMULATION OF BRANCHED AND CROSSLINKED POLYMERS

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.

Molecular simulation of polymeric networks and gels: phase behavior and swelling

Abstract Polymer gels are commonly used in industrial, analytical, and domestic applications; their uses are likely to continue expanding as gels with novel chemical and structural characteristics are developed. These applications often rely on the precise control of the adsorption behavior of a gel. Development of useful gels, however, has been hampered by a lack of molecular-level understanding of the physics underlying phase transitions in such materials. In this report, we review recent molecular simulation work related to the study of fundamental aspects of network elasticity and of phase transitions in polymeric gels. In particular, simulations of simplified (coarse-grained) molecular models are described which provide insights into the general behavior of gels, as opposed to studies concerned with the properties of specific materials. Methodological aspects unique to the simulation of different properties of polymeric gels are emphasized. We also pay special attention to the role of entropic factors (such as network topology, backbone stiffness, chain length asymmetry), over that of energetic interactions (such as hydrofobic interactions or ionic forces) on the onset and characteristics of phase transitions in gels. In spite of the important advances made over the last years in methodology and computer hardware, many challenges remain if phase transitions for more realistic gel models are to be simulated.