  We report variational and fixed-node diffusion quantum Monte Carlo (QMC)
calculations of anti-ferromagnetic iron oxide (FeO) in the ground state B1
crystal structure. The goal of this study was a systematic investigation of the
sensitivity of several ground state properties to a variety of QMC wave
function generation techniques including advanced wave functions such as
multi-determinant expansions and backflow transformations. We found that the
predicted lattice distortion was largely controlled by the choice of single
particle orbitals used to construct the wave function, rather than by
subsequent wave function optimization techniques within QMC. However, the
absolute magnetic moment was remarkably insensitive to the method of wave
function construction. QMC estimates of total spin density indicate that in
addition to strong electronic correlation of the Fe $3d$ states, charge
transfer may be an important but challenging piece of physics to accurately
capture within existing QMC methods. Finally, we highlight the need for
advanced and systematically improvable many-body wave functions suitable for
accurately describing challenging real systems.
