The proton paramagnetic relaxation enhancement (PRE) in an aqueous solution of nickel(II) is described in terms of electron spin relaxation caused by damped vibrational motions of E-g and T-2g symmetry. The damped vibrations generate a transient zero-field splitting (ZFS), of variable amplitude and variable principal direction in the molecular frame, and are modeled by the Smoluchowski equation. The parameters of the model are obtained from a combination of two approaches: first, quantum-chemical calculations of the ZFS as a function of the geometry of the coordination shell of the nickel(II) ion and, second, molecular-dynamic simulations generating a trajectory of water positions around the metal. The description of the electron spin dynamics is included in the calculations of the PRE in two ways: Using the traditional Solomon-Bloembergen-Morgan approach and also by means of the more general slow-motion theory. The calculated PRE as a function of the magnetic field, free of any adjustable parameters, is compared with the experimental data. The two methods of calculating the PRE agree with each other-and with the experimental data-at high magnetic field. At low field, the models predict very different PRE, and only the general model is in reasonable agreement with the experiments.