Langevin integrator with a collision frequency of 5 ps -1 and timestep of 5 fs was used. We chose this slightly conservative time step to avoid instabilities that may arise when exploring wide ranges of force field parameters. Replica exchange moves were attempted between neighboring temperature pairs after every 200 timesteps (1 ps), which we found to provide a good balance between computational efficiency and replica sampling efficiency. It should be noted that openmmtools supports a variety of other integrators, including Metropolized variants, in which the molecular dynamics segments are themselves subject to acceptance or rejection. In a preliminary replica exchange simulation 50 ns in duration, logarithmic temperature spacing was used. We then applied the constant entropy increase method as outlined above to the preliminary heat capacity data to refine the temperature spacing for a 200 ns production REMD simulation. To solve for the constant entropy increase temperature spacings, the C V T data was splined such that arbitrary definite integrals could be accurately evaluated. The temperature spacings were then optimized to minimize the standard deviation of the entropy increases, with the constraint that the temperature range be the same as the original. A comparison of replica exchange transition matrices from simulations with logarithmic temperature spacing and optimized CEI spacing is shown for an example system in the supporting information (Figure ).