  In this contribution we present calculations performed for interacting
electron systems within a non-perturbative formulation of the cluster theory.
Extrapolation of the model to describe the time dependence of the interacting
systems is feasible and planed. The theory is based on the unitary operator
$e^{iS}$ ({\it S} is the correlation operator) formalism which, in this paper,
is treated non perturbatively within many-particle correlations. The
application of the derived equations to few-body systems is realized in terms
of Generalized Linearization Approximations (GLA) and via the Cluster
Factorization Theory (CFT). To check the reliability of the model we present
two different applications. In the first we evaluate the transitions energies
in Helium-, Lithium-, Beryllium-, and Boron-like Oxygen. The calculation aims
to a precise determination of the satellite transitions which play an important
role in plasma diagnostics. In a second we investigate a non-perturbative
method to evaluate the charge radii of the Helium and Lithium isotopes by using
the Isotopic Shift theory. We have found that our model leads naturally to
components of $e^--e^+$ pair in the two-electron wave functions of the Helium
isotopes and three-electron wave functions of the Lithium isotopes. The
possible connection of these terms to the QED leading diagrams is postulated.
