First principles constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund's exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and corresponding bands are included, computation results in U=3-4 eV, however, with the basis set restricted to Fe-3d orbitals and bands only, computation gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local Density Approximation combined with the Dynamical Mean-Field Theory) calculation with this parameters results in weakly correlated electronic structure that is in agreement with X-ray experimental spectra.
@article{arxiv.0807.0547,
title = {Coulomb Parameter U and Correlation Strength in LaFeAsO},
author = {V. I. Anisimov and Dm. M. Korotin and S. V. Streltsov and A. V. Kozhevnikov and J. Kuneš and A. O. Shorikov and M. A. Korotin},
journal= {arXiv preprint arXiv:0807.0547},
year = {2009}
}