  In cold and shielded environments, molecules freeze out on dust grain
surfaces to form ices such as H2O, CO, CO2, CH4, CH3OH, and NH3. In
protoplanetary disks, the exact radial and vertical ice extension depend on
disk mass, geometry, and stellar UV irradiation. The goal of this work is to
present a computationally efficient method to compute ice and bare-grain
opacities in protoplanetary disk models consistently with the chemistry and to
investigate the effect of ice opacities on the physico-chemical state and
optical appearance of the disk. A matrix of Mie efficiencies is pre-calculated
for different ice species and thicknesses, from which the position dependent
opacities of icy grains are then interpolated. This is implemented in the
PRODIMO code by a self-consistent solution of ice opacities and the local
composition of ices, which are obtained from our chemical network. Locally, the
opacity can change significantly, for example, an increase by a factor of more
than 200 in the midplane, especially at UV and optical wavelengths, due to ice
formation. This is mainly due to changes in the size distribution of dust
grains resulting from ice formation. However, since the opacity only changes in
the optically thick regions of the disk, the thermal disk structure does not
change significantly. For the same reason, the spectral energy distributions
computed with our disk models with ice opacities generally show only faint ice
emission features at far-IR wavelengths. The ice absorption features are only
seen in the edgeon orientation. The assumption made on how the ice is
distributed across the grain size distribution influences the far-IR and
millimeter slope of the SED. The ice features and their strengths are
influenced by the ice power law and the type of chemistry. Our models predict
stronger ice features for observations that can spatially resolve the disk,
particularly in absorption.
