A familiar example of a rare event in this context would be nucleation of a raindrop from over-saturated water vapour: although raindrops form every day, relative to the length and time scales defined by the motion of water molecules in the vapour phase, the formation of a liquid droplet is extremely rare.
Due to the wide use of computer simulation across very different domains, articles on the topic arise from quite disparate sources and it is difficult to make a coherent survey of rare event sampling techniques.
[13][14] The first published rare event technique was by Herman Kahn and Theodore Edward Harris in 1951,[15] who in turn referred to an unpublished technical report by John von Neumann and Stanislaw Ulam.
In order to follow the time evolution of the probability of a rare event, it is necessary to maintain a steady current of trajectories into the target region of configurational space.
In cases where a dissipative steady state obtains (i.e. the conditions for thermodynamic equilibrium are not met, but the rare event flux is nonetheless constant) then FFS and other methods can be appropriate as well as the typically more expensive full-nonequilibrium approaches.