  The structure of the coronal magnetic field prior to eruptive processes and
the conditions for the onset of eruption are important issues that can be
addressed through studying the magnetohydrodynamic stability and evolution of
nonlinear force-free field (NLFFF) models. This paper uses data-constrained
NLFFF models of a solar active region that erupted on 2010 April 8 as initial
condition in MHD simulations. These models, constructed with the techniques of
flux rope insertion and magnetofrictional relaxation, include a stable, an
approximately marginally stable, and an unstable configuration. The simulations
confirm previous related results of magnetofrictional relaxation runs, in
particular that stable flux rope equilibria represent key features of the
observed pre-eruption coronal structure very well and that there is a limiting
value of the axial flux in the rope for the existence of stable NLFFF
equilibria. The specific limiting value is located within a tighter range, due
to the sharper discrimination between stability and instability by the MHD
description. The MHD treatment of the eruptive configuration yields very good
agreement with a number of observed features like the strongly inclined initial
rise path and the close temporal association between the coronal mass ejection
and the onset of flare reconnection. Minor differences occur in the velocity of
flare ribbon expansion and in the further evolution of the inclination; these
can be eliminated through refined simulations. We suggest that the slingshot
effect of horizontally bent flux in the source region of eruptions can
contribute significantly to the inclination of the rise direction. Finally, we
demonstrate that the onset criterion formulated in terms of a threshold value
for the axial flux in the rope corresponds very well to the threshold of the
torus instability in the considered active region.
