  Two improvements with respect to previous formulations are presented for the
calculation of the partition function $\mathcal{Z}$ of small, isolated and
interacting many body systems. By including anharmonicities and employing a
variational approach quantum effects can be treated even at very low
temperatures. A method is proposed of how to include collisional damping.
Finally, our approach is applied to the calculation of the decay rate of
metastable systems.
