Forward-Flux Sampling with Jumpy Order Parameters

Forward-flux sampling (FFS) [1] is a path sampling technique that has gained popularity in recent years, and has been used to compute rates of rare-event driven phenomena such as crystallization, condensation, hydrophobic evaporation and protein folding. The popularity of FFS is not only due to its ease of implementation, but also because of its lack of sensitivity to the particular choice of an order parameter. The order parameter utilized in conventional FFS, however, still needs to satisfy a stringent smoothness criterion in order to assure sequential crossing of FFS milestones. This condition is usually violated for order parameters utilized for describing aggregation phenomena such as crystallization. Here, we present a generalized FFS algorithm for which this smoothness criterion is no longer necessary, and apply it to compute homogeneous crystal nucleation rates in several systems. Our numerical tests reveal that conventional FFS can underestimate rate by several orders of magnitude.

[1] Allen, et al., J. Chem. Phys., 124: 194111 (2006).