MD Studies of LDL and HDL phases of Supercooled Water
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MD Studies of LDL and HDL phases of Supercooled Water
Xiaohai Li (ChBE) Vamsi Akkineni (PHYS) Jianwei Wang (GEOL)
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Project Goals
• Use Molecular Dynamics to simulate the LDL and HDL phases of supercooled water
• Obtain the radial distribution functions of two liquid phases of water
• Compare Results with literature results
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Motivation
• The liquid-liquid phase transition hypothesis is difficult to prove experimentally because of homogeneous necleation
• Computer simulation becomes extremely useful in this case because nucleation does not occur on the timescale of computer simulation
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The proposed phase diagram of supercooled water (O. Mishima, and H. E. Stanley, Nature 396, 329(1998))
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Intermolecular Potentials of water
• Interactions between water molecules are far more complicated than those between particles of simple liquids– The ability of water molecules to form hydrogen
bonds– The existence of substantial non-additive three-
and high-body terms
• The simulation are highly sensitive to the intermolecular potential functions used
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Intermolecular potentials of water
• Three kinds of intermolecular potentials
SPC TIP4P ST2
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SPC Potential
Charge on H: +0.41e and charge on O: -0.82e Angle of HOH: 109.47° Length of OH: 1.0Å = 3.166Å = 78.2 K kOH = 4637 kJ/(mol Å2) kHOH = 383 kJ/(mol rad2)
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MD Simulation
• Ensemble: NVT • Number of water molecules: 125• Use periodic boundry conditions • Time step: 0.001ps • Equilibration: 500ps • Collect data: 1000ps • Ewald summation used for long range Coulombic
forces. • Cutoff used for Lennard-Jones interactions (7Å)
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Snapshots of Two Systems
T=150k, d=0.95g/cm3 T=150k, d=1.20g/cm3
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(Huang et al.)
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Site-site radial distribution functions from EPSR simulations
(A. K. Soper, and M. A. Ricci, Phys. Rev. Lett. 84,2881(2000))
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Conclusion
• SPC model is suitable for simulating the supercooled water
• Our simulation supports the conjecture of LDL and HDL phases at low temperature
• More state points can be simulated to find the second critical point