3. Ultimately, populations and free energy differences between states drive the experimental measurements, so analysis ought to focus on state populations In this work, all possible tautomers of each ionization (charge) state are defined as distinct protonation microstates. For the pK a challenge, participants were required to report, for each molecule and each microstate they considered, the microstate ID of the reference state (selected by challenge organizers), the microstate ID of the microstate they were considering a transition to, the formal charge for the target microstate, and the predicted free energy change associated with a transition to the target microstate (Figure ), the relative free energy SEM, and the relative free energy model uncertainty. In many cases, the transitions to be considered were a particular physical reaction involving a change in a single protonation state or tautomer. However, in some cases transitions involved a change of multiple protons (e.g. the F-A transition of Figure ) and thus did not involve a single protonation or deprotonation event. Additionally, all transitions were defined as away from the reference state (and thus some involve gaining a proton, the opposite of a typical acid dissociation event), a point which caused confusion for a number of participants.