Second Renormalization of Tensor-Network States
  We propose a second renormalization group method to handle the tensor-network states or models. This method reduces dramatically the truncation error of the tensor renormalization group. It allows physical quantities of classical tensor-network models or tensor-network ground states of quantum systems to be accurately and efficiently determined. 
Z. Y. Xie, H. C. Jiang, Q. N. Chen, Z. Y. Weng, T. Xiang