Semiquantal approach to finite systems of interacting particles

A novel approach is suggested for the statistical description of quantum systems of interacting particles. We show that the occupation numbers for single-particle states can be represented as a convolution of a classical analog of the eigenstate, with the quantum occupation number for noninteracting particles. The latter takes into account the wave function symmetry and depends on the unperturbed energy spectrum only. As a result, the distribution of occupation numbers n(s) can be found even for a large number of interacting particles. Using the model of interacting spins, we demonstrate that this approach gives a correct description of n(s) even in deep quantum regions with few single-particle orbitals.