Spectral densities and partition functions of modular quantum systems as   derived from a central limit theorem
  Using a central limit theorem for arrays of interacting quantum systems, we give analytical expressions for the density of states and the partition function at finite temperature of such a system, which are valid in the limit of infinite number of subsystems. Even for only small numbers of subsystems we find good accordance with some known, exact results. 
Michael Hartmann, Guenter Mahler, Ortwin Hess