Thermodynamics of the anisotropic Heisenberg chain calculated by the density matrix renormalization group method

The density matrix renormalization group method is applied to the anisotropic Heisenberg chain at finite temperatures. The free energy of the system is obtained by using quantum transfer matrix which is iteratively enlarged in the imaginary time direction. The magnetic susceptibility and the specific heat are calculated down to $T=0.01J$ and compared with the Bethe ansatz results. The agreement is fairly good including the logarithmic correction in the magnetic susceptibility at the isotropic point.