Starting from the ACN case (acetonitrile has β = 0.40), a projection of the FES on the d variable clearly shows two main minima close to the recognition sites, at |d| ∼ 1.5 -2 nm from the axle center (Fig. , top: ∆G, black line). The free-energy difference ∆G between the bound minima and the unbound state in this system is found ∼ 11 -12 kcal mol -1 .