Here, we have presented smirnoff-plugins as a flexible framework to extend OpenFF style force fields with custom functional forms. Historically, training a new small molecule transferable force field for condensed phase simulations has been prohibitive due to the sheer number of fitting parameters that are required to cover chemical space. Interfacing smirnoff-plugins with the OpenFF software stack allows us to assign force field parameters via direct chemical perception, which substantially reduces the number of redundant parameters to be re-fit (for example, the 167 rotatable torsion parameters of Sage compared to the 146K of OPLS3e ). Furthermore, we can make use of robust tools, such as ForceBalance and Evaluator 20 , to automatically curate and train against thousands of experimental and quantum mechanical reference data points.