Density-functional calculation of the Coulomb repulsion and correlation strength in superconducting LaFeAsO

Constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion $U$ and Hund's exchange $J$ parameters for Fe-$3d$ electrons in LaFeAsO. Results strongly depend on the basis set. When O-$2p$, As-$4p$, and Fe-$3d$ orbitals are included, computation results in $U$=3$\div$4 eV. With the basis set restricted to Fe-$3d$ orbitals only, computation gives parameters corresponding to $F^0$=0.8 eV, $J$=0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure.