2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/82242Two 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.4 RevTeX pages, two figuresNuclear TheoryMesoscale and Nanoscale PhysicsNuclear Experimenttext