An algorithm is described for counting exactly internal molecular energy levels. Difficulties in the concept of the density of states are reinvestigated. The influence of anharmonicity is shortly discussed and it is shown that reliable densities can only be obtained for low energies if anharmonicity constants are used.
Kemper, M. J. H., van Dijk, J. M. F., & Buck, H. M. (1978). A backtracking algorithm for exact counting of internal molecular energy levels. Chemical Physics Letters, 53(1), 121-124. https://doi.org/10.1016/0009-2614%2878%2980405-9, https://doi.org/10.1016/0009-2614(78)80405-9