  The metal-insulator transition in correlated electron systems, where electron
states transform from itinerant to localized, has been one of the central
themes of condensed matter physics for more than half a century. The
persistence of this question has been a consequence both of the intricacy of
the fundamental issues and the growing recognition of the complexities that
arise in real materials, even when strong repulsive interactions play the
primary role. The initial concept of Mott was based on the relative importance
of kinetic hopping (measured by the bandwidth) and on-site repulsion of
electrons. Real materials, however, have many additional degrees of freedom
that, as is recently attracting note, give rise to a rich variety of scenarios
for a ``Mott transition.'' Here we report results for the classic correlated
insulator MnO which reproduce a simultaneous moment collapse, volume collapse,
and metallization transition near the observed pressure, and identify the
mechanism as collapse of the magnetic moment due to increase of crystal field
splitting, rather than to variation in the bandwidth.
