  We show that the layered-structure BaCuS$_2$ is a moderately correlated
electron system in which the electronic structure of the CuS layer bears a
resemblance to those in both cuprates and iron-based superconductors.
Theoretical calculations reveal that the in-plane $d$-$p$ $\sigma^*$-bonding
bands are isolated near the Fermi level. As the energy separation between the
$d$ and $p$ orbitals are much smaller than those in cuprates and iron-based
superconductors, BaCuS$_2$ is expected to be moderately correlated. We suggest
that this material is an ideal system to study the competitive/collaborative
nature between two distinct superconducting pairing mechanisms, namely the
conventional BCS electron-phonon interaction and the electron-electron
correlation, which may be helpful to establish the elusive mechanism of
unconventional high-temperature superconductivity.
