Non-Bonded Parameters. The Absolv package was developed as part of this project and offers a simple API for computing the change in free energy upon transferring a solute from one solvent to another, or to vacuum in the case of solvation free energy calculations. It offers two routes to this end: standard equilibrium, and non-equilibrium switching, calculations. Most importantly, by using OpenMM as the MD engine, the framework allows for free energies to be computed using arbitrary functional forms implemented via the OpenMM custom non-bonded class . Users are free to define their force field functional form and any special treatment to be applied during the calculation such as the soft-core approach to the LJ potential as part of the calculation setup. Absolv is compatible with the OpenFF software stack making it straightforward to test new functional forms implemented via smirnoff-plugins. Example scripts used to set up the calculations are included in the Supporting Data. To ensure the correctness of the default protocols offered by Absolv, regression tests were performed using systems previously studied to determine the reproducibility of free energy calculations between different software . For the nine free energy calculations run in triplicate we find the results from Absolv to be within error bars of the values reported by other packages (Figure ).