Extending the list of models given in Mandel et al. (1995); Fushman et al. (1997); Orekhov et al. (1999a); Korzhnev et al. (2001); Zhuravleva et al. (2004), the models built into relax include
The parameter Rex is scaled quadratically with field strength in these models as it is assumed to be fast. In the set theory notation, the model-free model for the spin system i is represented by the symbol