In the dipole-dipole mechanism, the TCF for the relaxation mechanism could be averaged over different interactions, which enhanced the signal-to-noise ratio. The approximations in the models for the dipole-dipole mechanism, were proven to be justified, and the MD simulations were in excellent agreement with experiment.
Most often the interaction potentials are derived by fitting the parameters to reproduce structural and some thermodynamical properties. In the same way, results from NMR studies can be used to calibrate the models used in MD simulations, allowing a refinement of potential models with respect to dynamical properties.
Time correlation functions from MD simulations can be Fourier transformed to power spectra if needed to provide line shapes and frequencies. This can be observed as multiple decay rates. Correlation functions give information about intermolecular interactions and reveal cases when several motional modes are contributing to relaxation mechanisms at slightly different time scales. This is useful in those cases when correlation times cannot be deduced from measurements of other isotopes in the same molecule or when there is no method available at all. MD simulations can be used to calculate both the correlation times and the whole correlation functions. MD can be used to evaluate the motional models and even replace them. MD simulations, can be considered as “model-free” in the sense that they do not assume the molecular motion to be in any specific regime. Since then simulations have been used in combination with both theory and experiment to develop new ideas, and MD simulations is becoming recognized as a vital tool for the understanding of the relaxation processes.Įxperimentalists often rely on motional models, based on hydrodynamics, in order to interpret their liquid state spectra. MD simulations of nuclear spin relaxation in liquids were initiated at a time when the development of theoretical models for many mechanisms was more or less stagnant. Michael Odelius, Aatto Laaksonen, in Theoretical and Computational Chemistry, 1999 5 CONCLUSIONS