Molecular simulation is an important tool to explore the behavior of molecules on the microscopic level. Through the principles of statistical mechanics one can relate this behavior to macroscopic properties, such as the vapor pressure, saturated liquid densities, or critical constants. A current limitation of this technique is that the intermolecular energies are usually described by functions ill-suited to systems that involve chemical reactions (the formation and destruction of chemical bonds). Kohn-Sham density functional theory provides a first-principles (that is, derived from quantum mechanics) description of the electrons and nuclei of the system, which in turn eliminates the need to specify the bonding system, required to study reactive systems, albeit at a significantly higher computational expense. A novel combination of advanced Monte Carlo simulation algorithms, highly efficient algorithms to solve the Kohn-Sham equations, and the massive parallel processing power at Lawrence Livermore National Laboratory have allowed the authors to study the vapor-liquid phase equilibrium of water and the vapor-phase aggregation of hydrogen fluoride, the results of which are presented here.
