Within a nonperturbative dynamical two-body approach - based on coupled
equations of motion for the one-body density matrix and the two-body
correlation function - we study the distribution of occupation numbers in a
correlated system close to the groundstate, the relaxation of single-particle
excitations and the damping of collective modes. For this purpose the nonlinear
equations of motion are solved numerically within a finite oscillator basis for
the first time adopting short-range repulsive and long-range attractive
two-body forces. We find in all cases that the formation of long- and
short-range correlations and their mixing is related to the long- and
short-range part of the nucleon-nucleon interaction which dominate the
resummation of loop or ladder diagrams, respectively. However, the proper
description of relaxation or damping phenomena is found to require both types
of diagrams as well as the mixed terms simultaneously.Comment: 31 LATEX pages, 18 figures available upon request, UGI-93-1