  Electronic structure of the manganese oxide solid is studied by the quantum
Monte Carlo (QMC) methods. The trial wavefunctions are built using orbitals
from unrestricted Hartree-Fock and Density Functional Theory, and the
electron-electron correlation is recovered by the fixed-node QMC. The
correlation effects are significant and QMC estimations of the gap and cohesion
show a very good agreement with experiment. Comparison with hybrid functional
results points out the importance of the exact exchange for improvement of the
Density Functional description of transition metal oxide systems.
