On the accuracy of the state space restriction approximation for spin   dynamics simulations
  We present an algebraic foundation for the state space restriction approximation in spin dynamics simulations and derive applicability criteria as well as minimal basis set requirements for practically encountered simulation tasks. The results are illustrated with NMR, ESR, DNP and Spin Chemistry simulations. It is demonstrated that state space restriction yields accurate results in systems where the time scale of spin relaxation processes approximately matches the time scale of the experiment. Rigorous error bounds and basis set requirements are derived. 
Alexander Karabanov, Ilya Kuprov, Gareth Charnock, Anniek van der   Drift, Luke Edwards, Walter K\"ockenberger
quant-ph math-ph math.MP