Redfield limit and beyond

The relaxation equation derived so far for electrons and nuclei share a common assumption usually called the perturbation regime or Redfield limit [54]. The 

In practice nuclear spin-lattice relaxation is always within the Redfield limit, i.e. the interaction energy with the lattice is always much smaller than rc-1. This is true even with paramagnetic systems, where the nuclear spin-lattice interaction eneigy is often much larger than usual. On the other hand, it is not obvious that electrons are always in the Redfield limit. When electrons are outside the Redfield limit, although nuclear relaxation is in the Redfield limit, it is not easy 

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