  Occupation numbers of natural orbitals capture the physics of strong electron
correlations in momentum space. A Natural Orbital Density Functional Theory
based on the antisymmetrized geminal product provides these occupation numbers
and the corresponding electron momentum density. A practical implementation of
this theory approximates the natural orbitals by the Kohn-Sham orbitals and
uses a mean-field approach to estimate pairing amplitudes leading to
corrections for the independent particle model. The method is applied to weakly
doped $\mbox{La$_2$CuO$_4$}$.
