Simulation of Phase Equilibria from First Principles: Applications to Water

Matthew McGrath, University of Minnesota

“In time and with water, everything changes.”

—Leonardo DaVinci


Water is the medium in which a multitude of reactions essential to life as we know it occurs. Because of this, water is probably the most studied liquid on the planet. For the past 40 years, computers have been used in an attempt to explore the microscopic structure of water in many environments, from high-density ice to atmospheric clusters. Despite this effort (and considerable progress), empirical models are unable to reproduce all chemical and thermophysical properties of this deceptively simple molecule.

The shortcomings of empirical models have led simulators to turn to a quantum mechanical description of the interaction energy in an effort to find more accurate water models. Here we use a series of extensive computer simulations to investigate structural, thermodynamic, and electronic properties of ab initio water over a range of state points. These properties include pair correlation functions and hydrogen bond populations of the liquid phase, saturated liquid and vapor densities, saturated vapor pressures, and heats of vaporization. The results of these simulations show decreased liquid densities, a lower critical temperature, and either increased or decreased structure (depending on the thermodynamic constraints) when compared to experiment. Additionally, at atmospheric pressure this particular variety of water was found to boil at a temperature of about 350 K, a few degrees K below the pure water found in most chemistry labs.

Abstract Author(s): Matthew J. McGrath, J. Ilja Siepmann, I.-Feng William Kuo, and Christopher J. Mundy