  A phenomenological method based on the natural orbital representation is
applied to construct the ground state one-body density matrix which describes
correctly both density and momentum distributions in $^{4}He$, $^{16}O$ and
$^{40}Ca$ nuclei. The parameters of the matrix are fixed by a best fit to the
experimental density distribution and to the correlated nucleon momentum
distribution. The method allows the natural orbitals, the occupation
probabilities and the depletion of the Fermi sea to be obtained. Ground-state
characteristics of $^{4}He$, $^{16}O$ and $^{40}Ca$ nuclei, such as rms radii
and mean kinetic energies are calculated, as well.
