Phase diagram of silica from computer simulation

We evaluate the phase diagram of the ``BKS'' potential [Van Beest, Kramer and van Santen, Phys. Rev. Lett. 64, 1955 (1990)], a model of silica widely used in molecular dynamics (MD) simulations. We conduct MD simulations of the liquid, and three crystals (beta-quartz, coesite and stishovite) over wide ranges of temperature and density, and evaluate the total Gibbs free energy of each phase. The phase boundaries are determined by the intersection of these free energy surfaces. Not unexpectedly for a classical pair potential, our results reveal quantitative discrepancies between the locations of the BKS and real silica phase boundaries. At the same time, we find that the topology of the real phase diagram is reproduced, confirming that the BKS model provides a satisfactory qualitative description of a silica-like material. We also compare the phase boundaries with the locations of liquid-state thermodynamic anomalies identified in previous studies of the BKS model.Comment: 7 pages, 7 figure

Understanding tetrahedral liquids through patchy colloids

We investigate the structural properties of a simple model for tetrahedral patchy colloids in which the patch width and the patch range can be tuned independently. For wide bond angles, a fully bonded network can be generated by standard Monte Carlo or molecular dynamics simulations of the model, providing a neat method for generating defect-free random tetrahedral networks. This offers the possibility of focusing on the role of the patch angular width on the structure of the fully bonded network. The analysis of the fully bonded configurations as a function of the bonding angle shows how the bonding angle controls the system compressibility, the strength of the pre-peak in the structure factor and ring size distribution. Comparison with models of liquid water and silica allows us to find the best mapping between these continuous potentials and the colloidal one. Building on previous studies focused on the connection between angular range and crystallization, the mapping makes it possible to shed new light on the glass-forming ability of network-forming tetrahedral liquids.Comment: 10 pages, 6 figure

Nucleation barriers in tetrahedral liquids spanning glassy and crystallizing regimes

Crystallization and vitrification of tetrahedral liquids are important both from a fundamental and a technological point of view. Here, we study via extensive umbrella sampling Monte Carlo computer simulations the nucleation barriers for a simple model for tetrahedral patchy particles in the regime where open tetrahedral crystal structures (namely cubic and hexagonal diamond and their stacking hybrids) are thermodynamically stable. We show that by changing the angular bond width, it is possible to move from a glass-forming model to a readily crystallizing model. From the shape of the barrier we infer the role of surface tension in the formation of the crystalline clusters. Studying the trends of the nucleation barriers with the temperature and the patch width, we are able to identify an optimal value of the patch size that leads to easy nucleation. Finally, we find that the nucleation barrier is the same, within our numerical precision, for both diamond crystals and for their stacking forms.Comment: 12 pages, 11 figure

Free energy surface of ST2 water near the liquid-liquid phase transition

We carry out umbrella sampling Monte Carlo simulations to evaluate the free energy surface of the ST2 model of water as a function two order parameters, the density and a bond-orientational order parameter. We approximate the long-range electrostatic interactions of the ST2 model using the reaction-field method. We focus on state points in the vicinity of the liquid-liquid critical point proposed for this model in earlier work. At temperatures below the predicted critical temperature we find two basins in the free energy surface, both of which have liquid-like bond orientational order, but differing in density. The pressure and temperature dependence of the shape of the free energy surface is consistent with the assignment of these two basins to the distinct low density and high density liquid phases previously predicted to occur in ST2 water.Comment: 8 pages, 9 figure

Nanodroplets and the equation of state of deeply supercooled water

We carry out extensive molecular dynamics simulations of nanoscale liquid droplets of the TIP4P/2005 model of water, with number of molecules ranging from N = 64 to 2880 and temperatures down to 180 K. As droplet size decreases, the Laplace pressure induced by the liquid-vapour surface tension increases. For sufficiently small droplets, the density within droplets exceeds the critical density associated with the liquid-liquid critical point proposed to occur deep in the supercooled region of the model. Since crystallization is suppressed for such small droplets, they provide a possible experimental probe for determining the equation of state for water where crystallization is otherwise unavoidable, and hence could provide direct evidence for the much-investigated second critical point scenario. However, it is unclear whether such small systems can provide any information on bulk water. We report on our progress in determining the relationships between N, temperature, pressure, and density, including the emergence of anomalous behaviour emblematic of bulk liquid water