Ab initio study of the low-lying electronic states of the C₂^- anion

Six low-lying states (X²σ_g⁺, A²Π_u, B²σ_u⁺, ⁴σ_g⁺, ⁴Π_u and ⁴σ_u⁺) of C2- anion are studied by highly correlated ab initio calculations. The potential energy curves (PECs) are computed in the internuclear separation from 0.8 to 5.0 Å using the complete active space self-consistent field method, and then performing the internally contracted multireference configuration interaction with Davidson correction. Core-valence correlation and scalar relativistic effect are considered through employing the aug-cc-pcV5Z-dk basis set. The spin-orbit coupling of the A²Π_u state is also calculated using Breit-Pauli Hamiltonian. Based on the PECs derived from the high-level ab initio calculations, the spectroscopic parameters are obtained by fitting the ro-vibrational levels that are acquired by solving the ro-vibrational Schrödinger equation. These spectroscopic parameters, especially for low-lying doublet states, are in good agreement with the previously calculated and experimental results. Finally, the electronic transition dipole moment matrix elements, Franck-Condon factors, radiative lifetimes and oscillator strengths of A²Π_u-X2σg+andB2σu+-X2σg+ are calculated and discussed as well.