sites.
:* is the rate at which molecules attach to the nucleus.
:* is the Zeldovich factor, (named after Yakov Zeldovich) which gives the probability that a nucleus at the top of the barrier will go on to form the new phase, rather than dissolve.

This expression for the rate can be thought of as a product of two factors: the first, , is the number of nucleation sites multiplied by the probability that a nucleus of critical size has grown around it. It can be interpreted as the average,