Identifying physical clusters in bubble nucleation

We present a new approach to molecular simulation of bubble nucleation. Our approach does not involve any ad hoc criteria to define a bubble for a giv en instantaneous configuration of molecules. Instead, we explore the stocha stic evolution of a system chosen as a small part of the liquid phase by me ans of an isothermal-isobaric Monte Carlo simulation aided by the umbrella sampling technique. The physical clusters relevant to nucleation, bubbles i n the present case, emerge naturally as we attain a coarse-grained descript ion of this stochastic process by introducing proper order parameters, i.e. , the volume and the interaction potential of the system. Thus, the concept of cluster commonly employed to describe vapor to liquid nucleation is gen eralized naturally for the case of bubble nucleation. The method is applied to Lennard-Jones fluids to evaluate the free energy of bubble formation un der a moderate negative pressure. The interaction potential plays a similar role to that in vapor to liquid nucleation in that it characterizes the sp atial extent of the bubble. There is thus a unity in free energetics of vap or to liquid nucleation and of bubble nucleation. (C) 1999 American Institu te of Physics. [S0021-9606(99)00427-4].