For the five AMBER force fields we tested, their ability to accurately model chignolin is related to their historical development. AMBER99 29 severely overestimates the misfolded state population of chignolin, with a large reduced chi-squared value (Figure ). AMBER99 overstabilizes α-helical conformations, and understabilizes the β-hairpin and C-terminus regions of human ubiquitin over long trajectories. Identification of these problems led to the development of AMBER99SB, which was parameterized by fitting glycine and alanine tetrapeptides local minima, resulting in improved dihedral parameters and better agreement with experimental observables. to achieve better reproduction of secondary structure and NMR observables. For chignolin, however, AMBER14SB is less accurate than AMBER99SB, underestimating the native state population (∼35%), with the third-highest reduced chi-squared value (3).