  We present the results of electronic structure calculations for the
two-dimensional ferromagnet Rb_2CrCl_4. They are obtained by the augmented
spherical wave method as based on density functional theory and the local
density approximation. In agreement with experimental data Rb_2CrCl_4 is found
to be semiconducting and displays long-range ferromagnetic order of the
localized Cr 3d moments. The magnetic properties are almost independent of the
structural modifications arising from the Jahn-Teller instability, which leads
from the parent body-centered tetragonal K_2NiF_4 structure to a side-centered
orthorhombic lattice. In contrast, our calculations give evidence for a strong
response of the optical band gap to the corresponding structural changes.
