  We have developed a first-principles local ansatz wavefunction approach with
momentum-dependent variational parameters on the basis of the tight-binding
LDA+U Hamiltonian. The theory goes beyond the first-principles Gutzwiller
approach and quantitatively describes correlated electron systems. Using the
theory, we find that the momentum distribution function (MDF) bands of
paramagnetic bcc Fe along high-symmetry lines show a large deviation from the
Fermi-Dirac function for the $d$ electrons with $e_{g}$ symmetry and yield the
momentum-dependent mass enhancement factors. The calculated average mass
enhancement $m^{\ast}/m = 1.65$ is consistent with low-temperature specific
heat data as well as recent angle-resolved photoemission spectroscopy (ARPES)
data.
