Impurity corrections to the thermodynamics in spin chains using a   transfer-matrix DMRG method
  We use the density matrix renormalization group (DMRG) for transfer matrices to numerically calculate impurity corrections to thermodynamic properties. The method is applied to two impurity models in the spin-1/2 chain, namely a weak link in the chain and an external impurity spin. The numerical analysis confirms the field theory calculations and gives new results for the crossover behavior. 
Stefan Rommer and Sebastian Eggert