Folded proteins in solution exhibit two general types of motion. The first is overall rotational tumbling. The time scale of the molecular reorientation (tumbling) in aqueous solution is governed by the size and shape of the protein, the viscosity of the solution, and temperature, and it is typically nanoseconds. The second type of motion is the internal dynamics of the nuclei in the molecular reference frame. The local fields experienced by the protein nuclei due to magnetic fluctuations are modulated by both types of motion and lead to nuclear spin relaxation. Experimental relaxation rates thus provide information about protein dynamics. After separation of the overall rotational diffusion, this approach senses internal mobility. The most common approach for interpreting relaxation experiments is the Lipari-Szabo (LS) analytical model. In this "model-free" model, overall tumbling and internal dynamics are assumed to be statistically independent. This is for example the case if they occur on sufficiently different time scales. Then, the time correlation function (TCF) factorizes as