Low-lying states of MX2 (M = Ag, Au; X = Cl, Br and I) with coupled-cluster approaches: effect of the basis set, high level correlation and spin–orbit coupling

Abstract

Theoretical study of coinage metal dihalides is challenging due to their complex electron correlation and relativistic effects, particularly the spin–orbit coupling (SOC) effect. Furthermore, vibrational frequencies and characters of their low-lying states are affected by the Renner–Teller effect (RTE) and pseudo-Jahn–Teller effect (PJTE). The theoretical results depend on the basis set and how the electron correlation is treated and previous results even differ qualitatively in the ground state for some silver dihalides. The equation-of-motion coupled-cluster method at the singles and doubles level (EOM–CCSD) for ionization potentials is employed in this work to calculate electron affinities and vertical excitation energies of MX₂ (M = Ag, Au; X = Cl, Br and I) as well as structures, harmonic frequencies and adiabatic excitation energies for the lowest two states of these molecules. Effects of the higher level correlation, basis set as well as SOC on these results, particularly on the π_u bending mode frequencies of the ²Π_g state, are investigated. A new assignment for photodetachment spectroscopy of AuCl₂⁻ is proposed where contribution of the ²Δ_g state is considered. The ground state of AgCl₂ is calculated to be the ²B₂ state of the C_2v structure in scalar-relativistic (SR) calculations, while the SOC quenches the RTE and PJTE and the ground state becomes the ²Π_3/2g state of the D_∞h structure. As for the other molecules, the ground state is calculated to be the ²Π_g state in SR cases, and the ²Π_3/2g state with SOC. Our results indicate that a higher level electron correlation, large basis set and SOC are important to achieve reliable results for these molecules.