Real-space renormalization group study of the Hubbard model on a   non-bipartite lattice
  We present the real-space block renormalization group equations for fermion systems described by a Hubbard Hamiltonian on a triangular lattice with hexagonal blocks. The conditions that keep the equations from proliferation of the couplings are derived. Computational results are presented including the occurrence of a first-order metal-insulator transition at the critical value of $U/t \approx 12.5$. 
J. X. Wang, Sabre Kais, R. D. Levine