In the spirit of assigning simple but informative computational challenges, SAMPL has, since 2012, challenged computational chemists to predict the affinities of aqueous host-guest systems , for hosts that include cucurbiturils , deep cavity cavitands , cyclodextrins (CDs), and derivatives thereof. Host-guest binding is both driven and opposed by the same basic forces the determine the affinities of protein-ligand systems, but they can be chosen to avoid ambiguities in protonation states that frequently arise in protein-ligand modeling, and they avoid the difficulties of ensuring adequate numerical convergence that are endemic in protein simulations. As a consequence, they can report clearly on the quality of the potential function, or force field, used in the calculations. Accordingly, recent studies have demonstrated the use of host-guest binding data to test and adjust the parameters of force fields, in order to enhance the accuracy of binding affinity calculations .