On the Entropy of Quantum Fields in Black Hole Backgrounds
  We show that the partition function for a scalar field in a static spacetime background can be expressed as a functional integral in the corresponding optical space, and point out that the difference between this and the functional integral in the original metric is a Liouville type action. A general formula for the free energy is derived in the high temperature approximation and applied to various cases. In particular we find that thermodynamics in the extremal Reissner-Nordstr\"om space has extra singularities that make it ill-defined. 
S. P. de Alwis and N. Ohta