  We present first-principles density-functional calculations using the local
density approximation to investigate the structural instability of cubic
perovskite BaBiO_3. This material might exhibit charge disproportionation and
some evidence thereof has been linked to the appearance of an additional,
fourth peak in the experimental IR spectrum. However, our results suggest that
the origin of this additional peak can be understood within the picture of a
simple structural instability. While the true instability consists of an
oxygen-octahedra breathing distortion and a small octahedra rotation, we find
that the breathing alone in a fcc-type cell doubling is sufficient to explain
the fourth peak in the IR spectrum. Our results show that the oscillator
strength of this particular mode is of the same order of magnitude as the other
three modes, in agreement with experiment.
