Electronic structure and bonding characters of the two lowest states of copper, silver, and gold monocarbides

The electronic structure and bonding characters of the two lowest states of CuC, AgC, and AuC have been systematically studied at Hartree-Fock (HF), coupled-cluster singles-doubles (CCSD), and coupled-cluster singles-doubles (triples) [CCSD(T)] levels. The ground states of CuC and AuC were assigned as 2Π while that of AgC assigned as 4Σ−. The differences in the interactions were observed, the combined effects of relativity and electron correlation from triples excitations are crucial to the 2Π state of CuC and those of relativity and electron correlation are important for the 2Π state of AuC to be the ground state. The binding characters of the 2Π states of CuC, AgC, and AuC are the genuine σ and the delocalized π bonds and that of the 4Σ− state is the strong σ bond. In the 2Π states electron correlation and relativistic effects hamper electron transfer along σ-frame and facilitate it along π-frame, while in the 4Σ− states electron transfer along σ-frame is hindered by relativistic effects and the influence by electron correlation effects is slight. In addition, the variation trends of spectroscopic parameters and electronic properties of each state caused by both effects were extracted and explained.