Real-Space Renormalization Group (RSRG) Approach to Quantum Spin Lattice   Systems
  The paper describes an explicit variational modification of the standard RSRG method and its application to quantum spin lattice systems. The modified approach is applied to exactly solvable ITF, XX and isotropic Heisenberg models. Better upper bounds for the systems' ground state energy are obtained as compared to the standard RSRG approach. 
A.S. Serov, G.V. Koval
cond-mat.str-el