Perturbation approach to dipolar coupling spin dynamics
  A perturbation method is presented which can be applied to the description of a wide range of physical problems that deal with dynamics of dipolar coupled spins in solids. The method is based on expansion of the operator exponent in a perturbation series. As example of application the method, the multile-quantum coherence dynamics in three and four spin cluster are considered. The calculated 0Q- and 2Q intensities vs the duration of the preparation period give closed agreement with exact results and simulations data. The exact solutions for J0Q and J2Q coherences in four spin system are obtained. 
G.B.Furman