Structural transitions in the 309-atom magic number Lennard-Jones cluster

The thermal behaviour of the 309-atom Lennard-Jones cluster, whose structure is a complete Mackay icosahedron, has been studied by parallel tempering Monte Carlo simulations. Surprisingly for a magic number cluster, the heat capacity shows a very pronounced peak before melting, which is attributed to several coincident structural transformation processes. The main transformation is somewhat akin to surface roughening, and involves a cooperative condensation of vacancies and adatoms that leads to the formation of pits and islands one or two layers thick on the Mackay icosahedron. The second transition in order of importance involves a whole scale transformation of the cluster structure, and leads to a diverse set of twinned structures that are assemblies of face-centred-cubic tetrahedra with 6 atoms along their edges, i.e., one atom more than the edges of the 20 tetrahedra that make up the 309-atom Mackay icosahedron. A surface reconstruction of the icosahedron from a Mackay to an anti-Mackay overlayer is also observed, but with a lower probability.Comment: 7 pages, 4 figure

Surface phase transitions and crystal habits of ice in the atmosphere

With climate modeling predicting a raise of at least 2°C by year 2100, the fate of ice has become a serious concern, but we still do not understand how ice grows (or melts). In the atmosphere, crystal growth rates of basal and prism facets exhibit an enigmatic temperature dependence and crossover up to three times in a range between 0° and −40°. Here, we use large-scale computer simulations to characterize the ice surface and identify a sequence of previously unidentified phase transitions on the main facets of ice crystallites. Unexpectedly, we find that as temperature is increased, the crystal surface transforms from a disordered phase with proliferation of steps to a smooth phase with small step density. This causes the anomalous increase of step free energies and provides the long sought explanation for the enigmatic crossover of snow crystal growth rates found in the atmosphere

Programming patchy particles to form three-dimensional dodecagonal quasicrystals

Model patchy particles have been shown to be able to form a wide variety of structures, including symmetric clusters, complex crystals and even two-dimensional quasicrystals. Here, we investigate whether we can design patchy particles that form three-dimensional quasicrystals, in particular targeting a quasicrystal with dodecagonal symmetry that is made up of stacks of two-dimensional quasicrystalline layers. We obtain two designs that are able to form such a dodecagonal quasicrystal in annealing simulations. The first is a one-component system of 7-patch particles but with wide patches that allow them to adopt both 7- and 8-coordinated environments. The second is a ternary system that contains a mixture of 7- and 8-patch particles, and is likely to be more realizable in experiments, for example, using DNA origami. One interesting feature of the first system is that the resulting quasicrystals very often contain a screw dislocation.Comment: 10 pages, 6 figure

Reversible self-assembly of patchy particles into monodisperse icosahedral clusters

We systematically study the design of simple patchy sphere models that reversibly self-assemble into monodisperse icosahedral clusters. We find that the optimal patch width is a compromise between structural specificity (the patches must be narrow enough to energetically select the desired clusters) and kinetic accessibility (they must be sufficiently wide to avoid kinetic traps). Similarly, for good yields the temperature must be low enough for the clusters to be thermodynamically stable, but the clusters must also have enough thermal energy to allow incorrectly formed bonds to be broken. Ordered clusters can form through a number of different dynamic pathways, including direct nucleation and indirect pathways involving large disordered intermediates. The latter pathway is related to a reentrant liquid-to-gas transition that occurs for intermediate patch widths upon lowering the temperature. We also find that the assembly process is robust to inaccurate patch placement up to a certain threshold, and that it is possible to replace the five discrete patches with a single ring patch with no significant loss in yield.Comment: 12 pages, 12 figure

A three dimensional integral equation approach for fluids under confinement: Argon in zeolites

Peer Reviewe

The phase diagram of water from quantum simulations

The phase diagram of water has been calculated for the TIP4PQ/2005 model, an empirical rigid non-polarisable model. The path integral Monte Carlo technique was used, permitting the incorporation of nuclear quantum effects. The coexistence lines were traced out using the Gibbs-Duhem integration method, once having calculated the free energies of the liquid and solid phases in the quantum limit, which were obtained via thermodynamic integration from the classical value by scaling the mass of the water molecule. The resulting phase diagram is qualitatively correct, being displaced to lower temperatures by 15-20K. It is found that the influence of nuclear quantum effects are correlated to the tetrahedral order parameter.Comment: 10 pages, 6 figures, 1 tabl

Self-assembly scenarios of patchy colloidal particles

The rapid progress in precisely designing the surface decoration of patchy colloidal particles offers a new, yet unexperienced freedom to create building entities for larger, more complex structures in soft matter systems. However, it is extremely difficult to predict the large variety of ordered equilibrium structures that these particles are able to undergo under the variation of external parameters, such as temperature or pressure. Here we show that, by a novel combination of two theoretical tools, it is indeed possible to predict the self-assembly scenario of patchy colloidal particles: on one hand, a reliable and efficient optimization tool based on ideas of evolutionary algorithms helps to identify the ordered equilibrium structures to be expected at T = 0; on the other hand, suitable simulation techniques allow to estimate via free energy calculations the phase diagram at finite temperature. With these powerful approaches we are able to identify the broad variety of emerging self-assembly scenarios for spherical colloids decorated by four patches and we investigate and discuss the stability of the crystal structures on modifying in a controlled way the tetrahedral arrangement of the patches.Comment: 11 pages, 7 figures, Soft Matter Communication (accepted

Rounded layering transitions on the surface of ice

Understanding the wetting properties of premelting films requires knowledge of the film’s equation of state, which is not usually available. Here we calculate the disjoining pressure curve of premelting films, and perform a detailed thermodynamic characterization of premelting behavior on ice. Analysis of the density profiles reveals the signature of weak layering phenomena, from one to two and from two to three water molecular layers. However, disjoining pressure curves, which closely follow expectations from a renormalized mean field liquid state theory, show that there are no layering phase transitions in the thermodynamic sense along the sublimation line. Instead, we find that transitions at mean field level are rounded due to capillary wave fluctuations. We see signatures that true first order layering transitions could arise at low temperatures, for pressures between the metastable line of water/vapor coexistence and the sublimation line. The extrapolation of the disjoining pressure curve above water vapor saturation displays a true first order phase transition from a thin to a thick film consistent with experimental observations