We use a set of ligands with cross-protein class inhibition against serine-and metallo-βlactamases, identified by Pemberton et al. It is comprised of heteroarylphosphonates with affinity for both a serine-β-lactamase (KPC-2) and two metallo-β-lactamases (NDM-1 and VIM-2). These ligands have a good dynamic range across the two enzyme classes and act as a good benchmark set for β-lactamases. For these ligands, we demonstrate that RBFE methods for serine-β-lactamases are accurate at predicting binding affinity, in line with other benchmarking. For VIM-2, we test a restraint-based method and an augmented parameter set for Amber-based zinc force fields using the same ligand force field parameters.