Accelerating simulation of metastable decay

We present a new adaptation of umbrella sampling technique in studying nucleation, allowing for a large free energy barrier to be overcome within a time scale accessible to simulation. The method requires a set of order parameters that can distinguish a metastable state and a more stable state toward which the system relaxes, but does not demand that the order parameters be adequate for describing the process around the top of the free energy barrier. To illustrate its effectiveness, the method was implemented using Monte Carlo simulation and was applied to study bubble nucleation in single component truncated and shifted Lennard-Jones fluids. Under the conditions studied, the rate of nucleation, measured in Monte Carlo cycle, was enhanced by a factor ranging from 10(2) to 10(7). The simulation also yields various thermodynamic and structural information on the system during nucleation.