The PDBbind database 31 is a curated set of bound macromolecule structures drawn from the Protein Data Bank (PDB) , each with an experimentally-measured binding affinity for its binding partner. Each release of PDBbind includes a "general set", which contains all the protein-ligand structures in the database; and a "refined set", a subset of protein-ligand complexes satisfying strict criteria concerning structure quality, affinity data reliability, and the nature of the complex. The 2018 release of PDBbind contains 16,151 protein-ligand complexes in the general set, with 4,463 complexes in the refined set. We used the refined set as our primary source of training data; however, it has been reported that including the lower-quality data comprising the remainder of the general set can still improve the performance of machine learning scoring functions , so we repeated our analysis using the general set as our source of training data.