In contrast to other domains, such as biomolecular systems, molecular dynamics (MD) simulations have been used relatively sparingly in solid systems. Although there was some scepticism that atomistic force fields would reproduce experimental behaviour in crystalline materials , there are now many successful examples of the application of MD to solids containing disorder. For example, MD has been applied to glassy molecular solids, showing that wellchosen force fields can accurately reproduce experimental C NMR shift distributions . Similarly, averaged NMR parameters derived from MD simulation have been used to understand the dynamics of molecules on catalytic surfaces , lipid membranes , and ionic liquids . Most applications to highly crystalline systems have involved porous or channel-containing host-guest systems. For example, the fast rotational and translational dynamics of urea inclusion systems can be effectively studied by conventional atomistic MD, see Ref. and references therein. Studies of gas absorption and diffusion in metal-organic frameworks use different approaches to simulation, notably Grand Canonical Monte Carlo simulations . A variety of methods have been used to study the dynamics of so-called "amphidynamic" crystals , solid systems which contain highly mobile components alongside a rigid molecular framework. Typically, the experimental rate information is derived from NMR, either 1 H T1 relaxation rates or 2 H NMR spectra . Such materials have been proposed as "molecular machines", e.g. crystalline molecular gyroscopes , in which a molecular fragment (a "rotor") is relatively free to rotate, or molecular gears , in which there is interaction between mobile components. Especially in the latter case, simulations of larger box sizes are needed to understand to the effects of correlation between different rotors . Correlated motions often require techniques such as metadynamics to enhance sampling beyond standard MD, and to improve the sampling of dynamical pathways on complex free energy surfaces.