FFS uses the same expression proposed by TIS for the calculation of the transition rate between 𝐴 and 𝐵, namely Eq: 12. However, it differs in the computational approach adopted for the calculation of the term 𝑃 (𝜆 𝑖+1 |𝜆 𝑖 ), a key difference that provides the attibute forward to the method's name. In FFS the probability of crossing interface 𝜆 𝑖+1 starting from interface 𝜆 𝑖 is obtained from the forward-only integration of the system's dynamics. This term is obtained as the fraction of trial runs initiated in 𝜆 𝑖 that reach 𝜆 𝑖+1 . Different implementations of the FFS algorithm differ in the specifics of the algorithm implemented to generate configurations at interfaces and thus in the details associated with the calculation of the 𝑃 (𝜆 𝑖+1 |𝜆 𝑖 ), and additional information can be found in the original publications and in a number of reviews covering the specifics of the method. Particularly important in the context of nucleation, where the size of the largest nucleus is typically a good choice of order parameter (see section 4.2.3), is the fact that jumpy order parameters require a specialized treatment in order to consistently yield estimates 𝑃 (𝜆 𝑖+1 |𝜆 𝑖 ) as discussed in detail by Haji-Akbari . Furthermore, Hall et al. provide a practical guide and comparison between the RETIS and FFS methods.