Accurate prediction of protein-ligand binding affinity is invaluable in the early stages of drug discovery. High-throughput alchemical free energy calculations are an attractive tool for this task, enabling rigorous calculation of binding free energies. However, accuracy remains limited by the description of interatomic interactions and sampling of conformational space. The potential energy surfaces of protein-ligand complexes are almost always described by molecular mechanics (MM) force fields, of which AMBER, OPLS, CHARMM and GROMOS are popular examples. These transferable biological force fields employ similar functional forms and their parameters are typically fit to reproduce the quantum mechanical (QM) and/or experimental properties of small organic molecules. Some alternative force fields, such as AMOEBA and QMDFF, have a greater focus on fitting to QM data, including symmetry-adapted intermolecular perturbation theory models, but typically have a more complex functional form and their use in protein-ligand binding studies are not routine.