  Alkali halides like Cesium Iodide have received renewed attention as a
material with potential application as detector coating due to the appearance
of Surface Plasmon Resonance absorption peak in their UV-visible absorption
spectrum. The formation of Surface Plasmon Resonance peak has been traced to
the formation of Cesium metal clusters due to the aglomeration of color centers
embedded in Cesium Iodide background. This paper, based on experimental
observation that cubic Cesium Iodide experiences stress due to color centers
forming in the neighborhood and take up tetragonal lattice structure,
investigates the tetragonal Cesium Iodide's band structure and dielectric
constant using ab initio calculations. Both, the band-gap and dielectric
constant of Cesium Iodide, show a systematic variation with changing lattice
constant. While the band-gap is seen to strongly depend on the lattice
parameter `c', the dielectric constant's variation along `a' direction (${\rm
0.2~nm^{-1}}$) was found to be stronger than that along the `c' direction
(${\rm -2.5~nm^{-1}}$). The calculated dielectric constants were used as input
parameter for Gan model calculations of the extinction coefficient and
evaluation of Surface Plasmon Resonance peak position (${\rm \lambda_{max}}$).
A red-shift in ${\rm \lambda_{max}}$ was seen with increasing dielectric
constant that appears with stress acting on Cesium Iodide's lattice.
