Both protein force fields yielded very comparable and mutually consistent results, despite the fact that the CHARMM36 force field was originally parametrized together with the TIP3P water model. Additionally, a near-quantitative agreement between experimental and computational cross-correlated relaxation rates was obtained, indicating that both force fields are able to accurately describe the nanosecond time scale dynamics of the side chains in ubiquitin. From a more general perspective, this work contributes to the ongoing efforts to systematically improve the computational description of not only the structures, but also of the structural dynamics of proteins. Improving the predictive capacity of computer simulations in this respect is a formidable challenge and a key requirement for ultimately predicting and quantitatively interpreting experiments. (32) Zhang, X.; Sui, X.; Yang, D. Probing methyl dynamics from Freiburg im Breisgau, Germany E-mail: lars.schaefer@ruhr-uni-bochum.de; fmulder@chem.au.dk