MRCI study of spectroscopic and molecular properties of X¹\varSigma_g⁺ and A¹\varPi_u electronic states of the C₂ radical

The potential energy curves (PECs) of X^1\varSigma _\rm g^ + and A^1\varPi_\rm u 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 (^12C_2, ^12C^13C and ^13C_2) have been determined. Compared in detail with previous studies reported in the literature, excellent agreement has been found. The complete vibrational levels G(\upsilon), inertial rotation constants B_\upsilon and centrifugal distortion constants D_\upsilon for the ^12C_2, ^12C^13C and ^13C_2 isotopologues have been calculated for the first time for the X^1\varSigma _\rm g^ + and A^1\varPi_\rm u 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.