The total energy of CaReN3 was calculated using the Quantum Espresso Simulation Package with the Perdew-Burke-Ernzerhof (PBE) wave function and 500 eV as a cut-off energy. The material posses a tetragonal shaped perovskite structure at 300K with the space group A2/n-15, consisting of Re-Ca bonds and a Ca-Re-N group. Optimization of cell characteristics along with atomic position is done. The band gap of CaReN3, calculated using DFT-GGA-PBE, was found to be around 1.8 eV. The relativistic effects of the Re and Ca atoms significantly impact the band gap, which is typically underestimated by normal DFT calculations. In the range of UV, the CaReN3 element used in optoelectronic devices due to its strong optical conductivity, low emissivity with high coefficient of absorption and raised refractive index. Less thean the band gap energy it exhibits transparent behaviour due to the incoming photon not having enough energy to move electrons between valence band and conduction band. The reflectivity spectra of CaReN3 show that it has higher reflectivity than other semiconductors, with the highest peaks occurring in the energy gap of 2.5 eV to 21 eV.