In equilibrium free energy calculations (EQ) a separate molecular dynamics (MD) simulation is performed for each of a set of discrete λ windows corresponding to different states of the system (Figure ). This method often requires long simulation times since every state has to first reach equilibrium before data can be used for the analysis. If this is not achieved -and it can be difficult to ensure that it has been achieved -the approach can suffer from systematic errors. In contrast, with a non-equilibrium approach (NEQ), only the physical end states (i.e. the interacting state and the noninteracting state) have to be sampled at equilibrium while the alchemical path is explored in fast switching transitions, as the ligand is switched from interacting to non-interacting (or vice versa) in a single simulation. The coupling parameter λ is continuous in this case. Multiple trials are performed, and the exponential average over the non-equilibrium work necessary to transform the ligand is related to the free energy difference of the process by the Jarzynski equality (or, better if transformations in both directions are possible, the Crooks formalism ).