Evaluating force fields by folding simulations.

The table shows whether we observed any folding events of villin and the WW domain in our simulations. A check mark indicates that simulations started in the unfolded state were able to reach the folded state in 10 µs (villin at 360 K) or 50 µs (WW domain at 370 K), while an “X” means that we did not observe any folding events. In those cases where we did observe at least one folding event, the numbers in parentheses indicate the number of folding/unfolding events we observed in that simulation. For ff99SB-ILDN, for example, we observed first a folding event and subsequently an unfolding event for villin, but only a folding event with no subsequent unfolding event for the WW domain. Since the simulations are roughly 10 times longer than the experimental folding and unfolding times, one would expect roughly five folding and five unfolding events in a force field that models perfectly both the kinetics and thermodynamics of folding.