Perturbation Expansion of the Partition Sum for any Temperature
  Based on the special properties of Liouville eigenoperators a perturbation theory for the partition sum is given. It is applicable for any temperature and includes the case of degenerate Hamiltonians. To demonstrate the reliability of the method, the second order correction to the atomic limit grand canonical potential of the Hubbard model is calculated and compared to results known from the literature. 
R. Schumann