The peak (in the total DOS) around -7.5 eV below the Fermi level mainly arises from the overlap of Zn 3d with Cu 4s/4p states. A broad and partially split peak at -2.5 to -5 eV below the Fermi level mainly arises due to the overlap between the Cu 3d states and Sb 5s/5p orbitals; as well as between the Cu 3d and Zn 4s/4p orbitals. These orbital interactions primarily lead to the significant reduction of the density of states across the Fermi level, from -0.5 eV to 0.5 eV.