Simulating the Homogeneous Liquid-Vapor Nucleation of Liquids In the Superheated and Stretched-Superheated Regimes Using Novel Molecular Dynamics Methods

Homogeneous liquid-vapor nucleation remains a process poorly understood at the microscopic level.  Estimations provided by classical nucleation theory over predict the free energy barrier and under predict the rate as conditions depart from liquid-vapor co-existence and thus do not accurately represent the transition.  Despite the practical importance of metastable liquids under negative pressures (e.g., as occurring in water transport inside trees), computational studies of homogeneous nucleation under such conditions have been scarce.  Recent simulation studies have examined either the rate or the free energy, but there is currently no consistent estimate for both values.  Using boxed molecular dynamics (BXD) we determine both the free energy barrier and the nucleation rate in a superheated Lennard-Jones fluid. Results of this study for such properties are consistent with those previously reported at similar or equivalent conditions. Complementary simulations using expanded-ensemble Monte Carlo methods and forward flux sampling (FFS) were also performed to validate the BXD results for the free energy and transition rates, respectively. Our study also extends to the application of both BXD and FFS to characterize the microscopic mechanism of bubble nucleation in a metastable liquid under tension, as the stability limit is approached.