The potential energy curves (PECs) of X 1 Σ + g and A 1 Πu electronic states of the C 2 radical have been studied 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 aug-cc-pV6Z basis set for internuclear separations from 0.08 nm to 1.66 nm. With these PECs of the C 2 radical, the spectroscopic parameters of three isotopologues ( 12 C 2 , 12 C 13 C and 13 C 2 ) have been determined. Compared in detail with previous studies reported in the literature, excellent agreement has been found. The complete vibrational levels G(υ), inertial rotation constants Bυ and centrifugal distortion constants Dυ for the 12 C 2 , 12 C 13 C and 13 C 2 isotopologues have been calculated for the first time for the X 1 Σ + g and A 1 Πu electronic states when the rotational quantum number J equals zero. The results are in excellent agreement with previous experimental data in the literature, which shows that the presented molecular constants in this paper are reliable and accurate.

Equilibrium internuclear separations, harmonic frequencies and potential energy curves (PECs) of HC1(X1Σ+) molecule are investigated by using the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with a series of correlation-consistent basis sets in the valence range. The PECs are all fitted to the Murrell–Sorbie function, and they are used to accurately derive the spectroscopic parameters (De, D0, ωeχe, αe and Be). Compared with the available measurements, the PEC obtained at the basis set, aug-cc-pV5Z, is selected to investigate the vibrational manifolds. The constants D0, De, Re, ωe, ωeχe, αe and Be at this basis set are 4.4006 eV, 4.5845 eV, 0.12757 nm, 2993.33 cm−1, 52.6273 cm−1, 0.2981 cm−1 and 10.5841 cm−1, respectively, which almost perfectly conform to the available experimental results. With the potential determined at the MRCI/aug-cc-pV5Z level of theory, by numerically solving the radial Schrödinger equation of nuclear motion in the adiabatic approximation, a total of 21 vibrational levels are predicted. Complete vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants are reproduced, which are in excellent agreement with the available Rydberg–Klein–Rees data. Most of these theoretical vibrational manifolds are reported for the first time to the best of our knowledge.