R. de Groot

Electrostatic interactions have been incorporated in dissipative particle dynamics (DPD) simulation. The electrostatic field is solved locally on a grid. Within this formalism, local inhomogeneities in the electrostatic permittivity can be treated without any problem. Key issues like the screening of the potential near a charged surface and the Stillinger–Lovett moment conditions are satisfied. This implies that the method captures the essential features of electrostatic interaction. For the direct simulation of mixed surfactants near oil–water interfaces, or for the simulation of Coulombic polymer–surfactant interactions, this method has all the advantages of DPD over full atomistic molecular dynamics (MD). DPD has proven to be faster than MD by many orders of magnitude, depending on the precise scaling factor chosen for the simulation. This brings phenomena of microseconds in reach of routine simulation, while maintaining a fairly accurate representation of the structure of the molecules. As an example of this simulation tool, the interaction between a cationic polyelectrolyte and anionic surfactant is discussed. Without a surfactant, the polyelectrolyte shows a fractal dimensionality that is in line with the theoretical and experimental values cited in literature, it behaves as a fairly stiff rod, df∼1.1. When salt is replaced by anionic surfactant, the polymer wraps around one or more discrete surfactant micelles, in line with the current understanding of these systems, and scaling invariance in the correlation function is broken.

We discuss the susceptibility of zinc-blende semiconductors to band-structure modification by insertion of small atoms at their tetrahedral interstitial sites. GaP is found to become an indirect-gap semiconductor with two He atoms present at its interstitial sites; Si does not. Analysis of the factors controlling these filling-induced electronic modifications allows us to predict that LiZnP [viewed as a zinc-blende-like (ZnP${)}^{\mathrm{\ensuremath{-}}}$ lattice partially filled with He-like ${\mathrm{Li}}^{+}$ interstitials], as well as other members of the Nowotny-Juza compounds ${A}^{\mathrm{I}{B}^{\mathrm{II}{O}^{\mathrm{V}}}}$, are likely to be a novel group of direct-gap semiconductors.

We present Gibbs ensemble Monte Carlo simulations of monomer–solvent and polymer–solvent mixtures with soft interaction potentials, that are used in dissipative particle dynamics simulations. From the simulated phase behavior of the monomer–solvent mixtures one can derive an effective Flory–Huggins χ-parameter as a function of the particle interaction potential. We show that this χ-parameter agrees very well with the free energy difference between a monomer surrounded by solvent particles, and a solvent particle surrounded by solvent particles. We develop a new “identity change” Monte Carlo move to equilibrate the polymer–solvent mixtures. In this move a polymer chain from one box is exchanged with an equal number of solvent particles from the other box. At realistic densities this new move offers a large computational advantage over the convential insertion method for a polymer chain using a configurational bias Monte Carlo algorithm. The new algorithm is demonstrated for polymer–solvent mixtures with a chain length of up to 150 segments. Significant differences are found between the simulated polymer–solvent phase behavior and results predicted by mean-field theory. Finally, we fit a master–equation to the simulated binodal curves at different chain lengths. This function is used to make a quantitative comparison between the simulations and experimental data for the phase equilibrium of the polystyrene–methylcyclohexane system.

The pair correlation function in a homogeneous hard sphere fluid at various densities has been measured in a large system, using the Monte Carlo method. The corresponding direct correlation function C2(r) has been determined directly from these measurements, and will be given here in closed form. We conclude that density functional models that neglect the effect of C3 and higher order direct correlation functions, that are defined in bulk fluids, are not able to describe an inhomogeneous hard sphere fluid accurately.