The Liquid-Liquid Phase Equilibrium Behavior of the ST2 Model of Water

Abstract

In this thesis, we were interested in investigating supercooled water, as represented by the ST2 model. The preponderance of evidence suggests there is a second critical point in supercooled water that produces two liquid phases, the high density liquid (HDL) phase and low density liquid (LDL) phase, at very cold temperatures. These temperatures pose considerable challenges to experimentation, so we investigated this through molecular dynamics simulations. First, we wanted to produce a phase diagram. Subsequently, we used the information contained in the phase diagram about the coexistence pressures and spinodal pressures to explore the relaxation dynamics of the system by finding the autocorrelation functions (ACFs) for the NPT ensemble for both density and Q6, a bond orientational parameter. We were able to compare the density and Q6 ACFs and see how the characteristic time changed as the spinodal was approached. In all these simulations, we also observed some interesting phenomenological occurrences including crystallization in the LDL phase for the NVT simulations, phase flipping near the coexistence pressure, and spontaneous phase changes between HDL and LDL near the spinodals in the NPT ensemble.