In the pK a challenge, participants submitted predictions consisting of the free energy changes between a reference microstate and every other relevant microstate for each compound. Specifically, participants were asked to predict the relative free energy between a selected neutral reference microstate and the rest of the enumerated microstates for that molecule at a reference pH of 0. In order to compare participants' predictions to experimental pK a values, these predicted relative free energies had to be converted to macroscopic pK a values.