Abstract
NMR spin-relaxation (in either small molecules or in biomolecules) can provide unique insights into to the time-dependence of conformational fluctuations, especially on picosecond to nanosecond time scales which can be directly probed by simulations. A great deal has been learned from molecular dynamics simulations about the general nature of such motions and their impact on NMR observables. In principle, relaxation measurements should also provide valuable benchmarks for judging the quantitative accuracy of simulations, and we survey recent progress in this direction, focusing on internal motions of protein backbones and side-chains, and overall rotational diffusion. Such information provides important pointers on how to carry out MD simulations with increased fidelity to the underlying structure and dynamics of the systems being studied.