ForceBalance 61 is a force field optimizer which constructs an objective function taking into account the deviations in MM estimated properties with respect to reference QM properties. A very large number of approaches are possible in weighting the different components of the fit from QM to MM structures. We describe below the current OpenFF approach, and how the parameters and justifications have evolved over time. Although other approaches to fitting may be possible and worth exploring, we have found that the approach reported here yields reasonable results that agree better-for both training and test sets-with benchmarks than our starting point force fields, and it performs better than any other fitting approach we have tried to date. However, we welcome experimentation from the scientific community, and our datasets and infrastructure are readily available to help facilitate such work. The optimization procedure used for Sage was similar to that used for Parsley, where the goal was to minimize the deviations in internal coordinates for optimized geometries and to minimize the deviations in relative energies with respect to a reference QM torsion profile. Each QM optimized geometry of a molecule is called an opt-geo target. Consideration of the opt-geo target involves evaluating a sum of the deviations in internal coordinates of MM optimized geometry at the current set of MM parameters at each iteration with respect to the QM reference. The QM torsional profile of a molecule, in which a specific central bond is rotated on a grid of dihedral angles, is called a torsion-profile target. Evaluating a torsion-profile target involves taking the differences in relative energies between a MM generated torsion profile at the current set of MM parameters, at each iteration, with respect to QM relative energies, at each of the grid points of the torsion scan.