  We present an analysis of the polymorphic energy ordering and properties of
the rock salt and zincblende structures of manganese oxide using fixed node
diffusion Monte Carlo (DMC). Manganese oxide is a correlated, antiferromagnetic
material that has proven to be challenging to model from first principles
across a variety of approaches. Unlike conventional density functional theory
and some hybrid functionals, fixed node diffusion Monte Carlo finds the rock
salt structure to be more stable than the zincblende structure, and thus
recovers the correct energy ordering. Analysis of the site-resolved charge
fluctuations of the wave functions according to DMC and other electronic
structure descriptions give insights into elements that are missing in other
theories. While the calculated band gaps within DMC are in agreement with
predictions that the zincblende polymorph has a lower band gap, the gaps
themselves overestimate reported experimental values.
