The potential energy curves (PECs) of six electronic states (B¹Δ_g, , C¹Π_g, , and 1¹Δ_u) of the C₂ radical have been investigated using the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the correlation-consistent aug-cc-pV6Z basis set for internuclear separations from 0.05 to 2.0 nm. The effects on the spectroscopic parameters (T_e , D ₀, D_e , R_e , ω_e , ω_ex_e , ω_ey_e , α_e , β_e , γ_e and B_e ) of the core–valence correlation and relativistic corrections are taken into account. The way to consider the relativistic corrections is to employ the second-order Douglas–Kroll Hamiltonian approximation. The core–valence correlation correction is performed at the cc-pCV5Z basis set. And the relativistic correction is carried out at the level of the cc-pV5Z basis set. In order to obtain reliable PECs, the Davidson modification is also included in the study. These spectroscopic parameters have been compared in detail with those of previous investigations reported in the literature, and excellent agreement has been found between the present results and the available experimental data. The first 20 vibrational states are computed for all these electronic states when the rotational quantum number J equals zero, and the vibrational levels, inertial rotation and centrifugal distortion constants of the B¹Δ_g and electronic states are reported when J = 0, which are in excellent agreement with the available measurements. Comparison with the available experimental data shows that the present molecular constants are both reliable and accurate.

The potential energy curves (PECs) of six low-lying electronic states (X²Π_g, a ⁴Π_u, A²Π_u, b ⁴ , D²Δ_g and B² ) of ion were studied by the ab initio quantum chemical method. The calculations were carried out with the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with large correlation-consistent basis sets. Effects on the PECs of the core–valence correlation and relativistic corrections are taken into account. The way to consider the relativistic correction is to use the second-order Douglas–Kroll Hamiltonian (DKH2) approximation. The core–valence correlation correction is carried out with the cc-pCVQZ basis set, and the relativistic correction is performed at the level of cc-pVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI + Q). These PECs are extrapolated to the complete basis set (CBS) limit by the two-point total-energy extrapolation scheme. With these PECs, the spectroscopic parameters (T_e , D_e , D ₀, R_e , ω_e , ω_ex_e , α_e and B_e ) are determined and compared with those reported in the literature. The conclusion can be reached that the effect on the spectroscopic parameters of the core–valence correlation correction is larger than that of the relativistic correction. With the PECs obtained by the MRCI + Q/CV+DK+56 calculations, the vibrational levels and inertial rotation constants of the first 26 vibrational states are determined for these electronic states of non-rotating ion. Comparison with the experimental data shows that the present spectroscopic parameters and molecular constants are accurate.