Natural orbital functional theory and pairing correlation effects in   electron momentum density
  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$}$. 
B. Barbiellini
cond-mat.str-el