  First-principles density functional theory calculations are performed to
understand the electronic structure and interaction parameters for recently
discovered superconducting Kagome metal CsV$_3$Sb$_5$. A systematic analysis of
the tight-binding parameters based on maximally localized Wannier function
method demonstrates that the out-of-plane Sb$^{\rm out}$-$p$ orbital is a key
element in complete description of the three Van Hove singularity structures
known in this material at $M$ point near the Fermi level. Further, the
correlation strengths are also largely determined by Sb$^{\rm out}$-$p$ states.
Based on constrained random phase approximation, we find that on-site and
inter-site interaction parameter are both significantly affected by the
screening effect of Sb$^{\rm out}$-$p$ orbitals. As the role of this previously
unnoticed orbital state can be tuned or controlled by out-of-plane lattice
parameters, we examine the electronic structure and particularly the evolution
of Van Hove singularity points as a function of strain and pressure, which can
serve as useful knobs to control the material properties.
