On the time evolution of fermionic occupation numbers.

We derive an approximate equation for the time evolution of the natural occupation numbers for fermionic systems. The evolution of such numbers is connected with the symmetry-adapted generalized Pauli exclusion principle, as well as with the evolution of the natural orbitals and a set of many-body relative phases. We then relate the evolution of these phases to a geometrical and a dynamical term attached to some of the Slater determinants appearing in the configuration-interaction expansion of the wave function. Our approach becomes exact for highly symmetric systems whenever the wave function possesses as many Slater determinants as independent occupation numbers.