We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real and reciprocal space data, a picture of a two-step crystallization process is supported: First dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that has no metastable fluid-fluid coexistence) crystallizes via the same mechanism.The crystallization process in complex fluids is not trivial. For systems such as solutions of proteins, alkanes, and colloids it has been shown that crystal nucleation rates can be enhanced considerably if the supersaturated liquid is quenched to a state that lies close to a metastable fluid-fluid critical point [1][2][3][4][5][6][7][8]. The enhanced nucleation rate is generally attributed to the fact that the density fluctuations occuring in the vicinity of a metastable fluid-fluid critical point enable the system to evolve via a two-step process. First dense, amorphous precursors form and then the crystallization process takes place inside these. The prerequisite of this process scenario, the metastable fluid-fluid critical point, is easily realized in the systems listed above, which exhibit an interplay of repulsive and attractive interactions.However, it is worthwhile asking whether the two-step process occurs more generally. Surprisingly, there have been experiments indicating two-step crystallization occuring also in hard sphere systems, the simplest model system for liquids and crystals (see e.g. Ref.[9]). As the interaction energy between two hard spheres is either zero (no overlap) or infinite (overlap), the phase behaviour of the system is purely determined by entropy. In particular, for one component hard spheres there exists a stable crystalline phase but no metastable fluid-fluid demixing region.The crystallization kinetics in colloidal hard sphere systems has been studied experimentally using predominantly time resolved light scattering [9][10][11][12][13][14][15] and to a lesser extend real-space imaging techniques [16][17][18][19]. In the scattering experiments described in Refs. [9,15,20] the time-evolution of the structure factor has been interpreted using a two-step process model: In the induction stage precursors (compressed, structurally heterogeneous clusters) slowly grow. Then the precursors are converted into highly ordered crystals in a fast, activated process. In Ref. [9] it was suggested that size polydispersity limited growth is responsible for the induction stage. However, later it was argued that the precursor stage behaves in a similar fashion, regardless of polydispersity or of metastability suggesting that the precurs...

The structure and growth of crystal nuclei that spontaneously form during computer simulations of the simplest nontrivial model of a liquid, the hard sphere system, is described in this work. Compact crystal nuclei are observed to form at densities within the coexistence region of the phase diagram. The nuclei possess a range of morphologies with a predominance of multiply twinned particles possessing in some cases a significant decahedral character. However the multiply twinned particles do not form from an initial decahedral core but appear to nucleate as blocks of a face-centered cubic crystal partially bounded by stacking faults.

We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework.

The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

Glassy states have been observed in hard-spherelike colloidal suspensions; however, some recent work suggests that a stable, one-component hard-sphere glass doesn't exist. A possible resolution of this dilemma is that colloidal glass formation results from a small degree of particle polydispersity. In order to investigate this further, we used the molecular-dynamics method to explore the phase behavior of both one- and two-component hard-sphere systems. It was found that the metastable fluid branch of the one-component system ceased to exist at a volume fraction marginally above melting, instead this system always crystallized within a relatively short period of time. Binary systems with a size ratio gamma=0.9 were then used as the simplest approximation to model a polydisperse hard-sphere colloidal system. Here the crystallization process was slowed down dramatically for all volume fractions and the fluid state was maintained for many relaxation times. Indeed, at the lowest volume fraction straight phi=0.55 no sign of crystallization was seen on the simulation time scale. The systems at intermediate volume fractions did eventually crystallize but at the highest volume fraction of straight phi=0.58, a dramatic slowing down in the crystallization process was observed. This is qualitatively in agreement with the experimental results on colloidal suspensions. Using the insight gained from this paper, the reasons behind a polydisperse system forming a stable glass, in contrast to the one-component system, are elucidated.