545 research outputs found
Phase diagram of the ST2 model of water
We evaluate the free energy of the fluid and crystal phases for the ST2
potential [F.H. Stillinger and A. Rahman, J. Chem. Phys. 60, 1545 (1974)] with
reaction field corrections for the long-range interactions. We estimate the
phase coexistence boundaries in the temperature-pressure plane, as well as the
gas-liquid critical point and gas-liquid coexistence conditions. Our study
frames the location of the previously identified liquid-liquid critical point
relative to the crystalline phase boundaries, and opens the way for exploring
crystal nucleation in a model where the metastable liquid-liquid critical point
is computationally accessible
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
Crystal Nucleation in a Supercooled Liquid with Glassy Dynamics
In simulations of supercooled, high-density liquid silica we study a range of
temperature T in which we find both crystal nucleation, as well as the
characteristic dynamics of a glass forming liquid, including a breakdown of the
Stokes-Einstein relation. We find that the liquid cannot be observed below a
homogeneous nucleation limit (HNL) at which the liquid crystallizes faster than
it can equilibrate. We show that the HNL would occur at lower T, and perhaps
not at all, if the Stokes-Einstein relation were obeyed, and hence that glassy
dynamics plays a central role in setting a crystallization limit on the liquid
state in this case. We also explore the relation of the HNL to the Kauzmann
temperature, and test for spinodal-like effects near the HNL.Comment: 4 pages, 4 figure
Density minimum and liquid-liquid phase transition
We present a high-resolution computer simulation study of the equation of
state of ST2 water, evaluating the liquid-state properties at 2718 state
points, and precisely locating the liquid-liquid critical point (LLCP)
occurring in this model. We are thereby able to reveal the interconnected set
of density anomalies, spinodal instabilities and response function extrema that
occur in the vicinity of a LLCP for the case of a realistic, off-lattice model
of a liquid with local tetrahedral order. In particular, we unambiguously
identify a density minimum in the liquid state, define its relationship to
other anomalies, and show that it arises due to the approach of the liquid
structure to a defect-free random tetrahedral network of hydrogen bonds.Comment: 5 pages, 4 figure
Spectral statistics of the quenched normal modes of a network-forming molecular liquid
We evaluate the density of states of the quenched normal modes of ST2 water,
and their statistical fluctuations, for a range of densities spanning three
regimes of behavior of a hydrogen bonded liquid: a lower-density regime of
random tetrahedral network formation; in the vicinity of a liquid-liquid
critical point; and in a higher-density regime of fragile glass-forming
behavior. For all cases we find that the fluctuations around the mean spectral
densities obey the predictions of the Gaussian orthogonal ensemble of random
matrix theory. We also measure the participation ratio of the normal modes
across the entire frequency range, and find behavior consistent with the
majority of modes being of an extended nature, rather than localized.Comment: Accepted for publication in The Journal of Chemical Physic
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