In this sequential approach, the first step is to create a set of conformations for both physical end-states (corresponding to λ = 0 and 1) by performing equilibrium simulations at each. Thereafter, fast transitions are initiated from each conformation of either end-state, transforming the system into the other end-state through the alchemical space. The transitions initiated from the λ = 0 end-state conformations are called "forward" transitions, whereas those initiated from the λ = 1 end-state conformations are termed "reverse" transitions. Such fast transitions drive the system out of equilibrium; hence it is a non-equilibrium approach. The dissipation work done to perform each transition is obtained by integrating ∂H/∂λ over the course of the transition (referred to as "work values" hereafter). ∆G values are computed from the work value distributions in both forward and reverse directions using a maximum likelihood estimator based on the Crooks Fluctuation Theorem 55 using an adapted version of pmx .