  The influence of correlations on the momentum distribution of nucleons in
nuclei is evaluated starting from a realistic nucleon-nucleon interaction. The
calculations are performed directly for the finite nucleus $\,^{16}$O making
use of the Green's function approach. The emphasis is focused on the
correlations induced by the excitation modes at low energies described within a
model-space of shell-model configurations including states up to the sdg shell.
Our analysis demonstrates that these long-range correlations do not produce any
significant enhancement of the momentum distribution at high missing momenta
and low missing energies. This is in agreement with high resolution $(e,e'p)$
experiments for this nucleus. We also try to simulate the corresponding effects
in large nuclei by quenching the energy-spacing between single-particle orbits.
This yields a sizable enhancement of the spectral function at large momenta and
small energy. Such behavior could explain the deviation of the momentum
distribution from the mean field prediction, which has been observed in
$(e,e'p)$ experiments on heavy nuclei like $^{208}$Pb.
