Tight-binding Hamiltonian for LaFeAsO

There have been several first-principles calculations reported recently for the superconducting pnictinide LaFeAsO and related compounds. In addition, tight-binding(TB) Hamiltonians for these systems have been constructed with varying degrees of success. In this work we have used the NRL-TB method to fit our LAPW results to a TB basis with the aim of reproducing the band structure very accurately. We have included the s and d orbitals of Fe, the s and p orbitals of As, and the p orbitals of O. We present a study of these TB results in terms of the effect of each of the above orbitals on how accurately the first-principles band structure can be reproduced. Finally, we assess the feasibility of carrying out many-body theory with a Hamiltonian that may contain more than just the d-Fe orbitals.