Nine force fields (AMBER14SB, 37 AMBER99, AMBER99SB, 5 AMBER99SB-ildn, 6 AMBER99SBnmr1ildn, 32 CHARMM22*, 12 CHARMM27, 38 CHARMM36 7 and OPLS-aa 39 ) were selected to represent a variety of approaches and historical improvements. All force fields are available for use with the GROMACS molecular simulation package. In order to construct comprehensive MSMs for simulations performed in each force field, several steps were performed to ensure model optimization and validity.