(1) where n f and n r are the number of switches in the forward and reverse directions, respectively, and ∆G is the free energy difference between the end states. The works along the forward and reverse pathways, W f and W r , are calculated by accumulating the energy changes as the coupling parameter λ is changed during the switches W = 1 0 δH(λ) δλ dλ , where H is the Hamiltonian of the system.