MD Studies of LDL and HDL phases of Supercooled Water

MD Studies of LDL and HDL phases of Supercooled Water

Xiaohai Li (ChBE) Vamsi Akkineni (PHYS) Jianwei Wang (GEOL)

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

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

The proposed phase diagram of supercooled water (O. Mishima, and H. E. Stanley, Nature 396, 329(1998))

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

Intermolecular potentials of water

• Three kinds of intermolecular potentials

SPC TIP4P ST2

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)

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)

Snapshots of Two Systems

T=150k, d=0.95g/cm3 T=150k, d=1.20g/cm3

(Huang et al.)

Site-site radial distribution functions from EPSR simulations

(A. K. Soper, and M. A. Ricci, Phys. Rev. Lett. 84,2881(2000))

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