2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149270The massive scalar field with $λφ^4$ interaction placed in $(3+1)$ dimensional box is considered. The sizes of the box are $V\times β$ $(V=L^3$ is the volume, $T=1/β$ is the temperature). The free energy is evaluated up to the 2nd order of $PT$. The averaging on the vacuum fluctuations is separated from the averaging on the thermal fluctuations explicitly. As result the free-energy is expressed through the scattering amplitudes. We find that in 3-loop approximation the expression for free energy coincides with the ansatz of Bernstein, Dashen, Ma suggested on the base of $S$-matrix formulation of statistical mechanics. The obtained expressions are generalized for higher order of $PT$.11 pages, LaTeX fileHigh Energy Physics - Theorytext