Systematic Mapping of the Hubbard Model to the Generalized t-J Model
  The generalized t-J model conserving the number of double occupancies is constructed from the Hubbard model at and in the vicinity of half-filling at strong coupling. The construction is realized by a self-similar continuous unitary transformation. The flow equation is closed by a truncation scheme based on the spatial range of processes. We analyze the conditions under which the t-J model can be set up and we find that it can only be defined for sufficiently large interaction. There, the parameters of the effective model are determined. 
Alexander Reischl, Erwin M\"uller-Hartmann, and G\"otz S. Uhrig