  Accretion disks around supermassive black holes have regions where the
Rosseland mean opacity can be much larger than the electron scattering opacity
primarily due to the large number of bound-bound transitions in iron. We study
the effects of this iron opacity "bump" on the thermal stability and vertical
structure of radiation pressure dominated accretion disks, utilizing three
dimensional radiation magneto-hydrodynamic simulations in the local shearing
box approximation. The simulations self-consistently calculate the heating due
to MHD turbulence caused by magneto-rotational instability and radiative
cooling by using the radiative transfer module based on a variable Eddington
tensor in Athena. For a $5\times 10^8$ solar mass black hole with $\sim 3\%$ of
the Eddington luminosity, a model including the iron opacity bump maintains its
structure for more than $10$ thermal times without showing significant signs of
thermal runaway. In contrast, if only electron scattering and free-free opacity
are included as in the standard thin disk model, the disk collapses on the
thermal time scale. The difference is caused by a combination of 1) an
anti-correlation between the total optical depth and the temperature, and 2)
enhanced vertical advective energy transport. These results suggest that the
iron opacity bump may have a strong impact on the stability and structure of
AGN accretion disks, and may contribute to a dependence of AGN properties on
metallicity. Since this opacity is relevant primarily in UV emitting regions of
the flow, it may help to explain discrepancies between observation and theory
that are unique to AGNs.
