Although we did not compare the direct and van't Hoff approaches for the present systems, it is clear that the direct calculations were slow to converge by current standards, as simulations of about 300µs duration still left us with uncertainties of 2-3 kcal/mol. Interestingly, though, the uncertainties were only this large for the protein-ligand systems. For the ligands alone, the convergence was excellent, even though the number of atoms in the free ligand systems was about the same as that in the protein-ligand systems. Thus, the problem was not the size of the proteinligand systems, but the occurrence of slow conformational changes, involving drift of the C-terminal part of the protein away from its crystallographic conformation. This drift away from the crystal structure might reflect a problem with the force field. However, examination of the crystal structures reveals another possible explanation, the existence of crystal contacts that involve the C-terminus and that might have stabilized a crystallographic conformation that becomes unstable in solution. There may well be other proteins of similar size, or even larger, where such slow motions away from the crystal structure do not occur, and for for which convergence of the direct method would, as a consequence, be substantially faster. Continued increases in computational speed, afforded by coprocessors like GPUs , and specialized computers like Anton 2 , may well allow calculations like these to be converged to within 1 kcal/mol uncertainty within the next few years.