In the case of "molecular machines", the motion of the fragment is well defined, and the challenge is to determine the rate of motion and how this is influenced by molecular interactions. In the case of pharmaceutical solvates, the nature of the guest molecule motion is undefined, and it is not obvious how to use molecular dynamics trajectories to derive a physical picture of the motion or compare to experiment. DMSO molecules have been observed to rotate through 180° in MD simulations of a DMSO solvate of carbamazepine, allowing the experimental crystallographic data to be rationalised . MD simulations have been used in a similar way to rationalise experimental data on the dynamics of pyridinium cations obtained from quasi-elastic neutron scattering , and F NMR relaxation data observing molecular motion in solid octafluoronaphalene .