Impurity corrections to the thermodynamics in spin chains using a transfer-matrix DMRG method

We use the density-matrix renormalization group 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 explicit results for the crossover behavior.