A long sought goal in the physical chemistry of macromolecular structure, and one directly relevant to understanding the molecular basis of biological recognition, is predicting the geometry of bimolecular complexes from the geometries of their free monomers. Even when the monomers remain relatively unchanged by complex formation, prediction has been difficult because the free energies of alternative conformations of the complex have been difficult to evaluate quickly and accurately. This has forced the use of incomplete target functions, which typically do no better than to provide tens of possible complexes with no way of choosing between them. Here we present a general framework for empirical free energy evaluation and report calculations, based on a relatively complete and easily executable free energy function, that indicate that the structures of complexes can be predicted accurately from the structures of monomers, including close sequence homologues. The calculations also suggest that the binding free energies themselves may be predicted with reasonable accuracy. The method is compared to an alternative formulation that has also been applied recently to the same data set. Both approaches promise to open new opportunities in macromolecular design and specificity modification.Keywords: desolvation free energy; electrostatic interaction energy; rigid body docking; side-chain conformational search Understanding biological function at its most fundamental level, and manipulating it for therapeutic purposes, requires understanding the molecular basis of specificity. The critical determinant of true understanding is the ability to make quantitative predictions. In the case of molecular recognition, this means predicting the geometry of a complex from the free structures of its constituents.Even when conformations remain relatively unperturbed by reaction, thereby restricting the search for potential molecular complexes to a manageable size, achieving the goal of true prediction has proved elusive. Rigid body docking algorithms (Cherfils et al., 1991;Jiang & Kim, 1991;Shoichet & Kuntz, 1991;Bacon & Moult, 1992;Stoddard & Koshland, 1992;Walls & Sternberg, 1992;Helmer-Citterich & Tramontano, 1994;Judson et al., 1995) permit exploration of the orientations of a ligand in a receptor combining site, and effectively select those relatively few that meet prespecified criteria-including surface complementarity, interaction energy, and hydrophobic surface burial.As shown by Shoichet and Kuntz (1991), the methods go far toward reducing hundreds of thousands of possibilities to tens ~