  In order to investigate electronic properties of recently discovered heavy
fermion superconductors CeTIn$_5$ (T=Ir and Co), we employ the relativistic
linear augmented-plane-wave (RLAPW) method to clarify the energy band
structures and Fermi surfaces of those materials. The obtained energy bands
mainly due to the large hybridization between Ce $4f$ and In 5$p$ states well
reproduce the Fermi surfaces consistent with the de Haas-van Alphen
experimental results. However, when we attempt to understand magnetism and
superconductivity in CeTIn$_5$ from the microscopic viewpoint, the energy bands
obtained in the RLAPW method are too complicated to analyze the system by
further including electron correlations. Thus, it is necessary to prepare a
more simplified model, keeping correctly the essential characters of the energy
bands obtained in the band-structure calculation. For the purpose, we construct
a tight-binding model for CeTIn$_5$ by including $f$-$f$ and $p$-$p$ hoppings
as well as $f$-$p$ hybridization, which are expressed by the Slater-Koster
integrals, determined by the direct comparison with the band-calculation
result. Similarity and difference between CeIrIn$_5$ and CeCoIn$_5$ are
discussed based on the obtained tight-binding model, suggesting a significant
importance of the effect of crystalline electric field to understand the
difference in electronic properties among CeTIn$_5$.
