Efficient evaluation of partition functions of frustrated and   inhomogeneous spin systems
  We present a numerical method to evaluate partition functions and associated correlation functions of inhomogeneous 2--D classical spin systems and 1--D quantum spin systems. The method is scalable and has a controlled error. We illustrate the algorithm by calculating the finite--temperature properties of bosonic particles in 1--D optical lattices, as realized in current experiments. 
V. Murg, F. Verstraete and J. I. Cirac