Accurate ab initio potential energy curves and spectroscopic properties of the four lowest singlet states of C2

The diatomic carbon molecule has a complex electronic structure with a large number of low-lying electronic excited states. In this work, the potential energy curves (PECs) of the four lowest lying singlet states

$$ \left( {X^1 \Sigma_g^ + ,\,A^1 \Pi _u ,\,B^1 \Delta_g ,\,and\,B'^1 \Sigma_g^ + } \right) $$

were obtained by high-level ab initio calculations. Valence electron correlation was accounted for by the correlation energy extrapolation by intrinsic scaling (CEEIS) method. Additional corrections to the PECs included core–valence correlation and relativistic effects. Spin–orbit corrections were found to be insignificant. The impact of using dynamically weighted reference wave functions in conjunction with CEEIS was examined and found to give indistinguishable results from the even weighted method. The PECs showed multiple curve crossings due to the

$$ B^1 \Delta_g $$

state as well as an avoided crossing between the two

$$ ^1 \Sigma_g^ + $$

states. Vibrational energy levels were computed for each of the four electronic states, as well as rotational constants and spectroscopic parameters. Comparison between the theoretical and experimental results showed excellent agreement overall. Equilibrium bond distances are reproduced to within 0.05 %. The dissociation energies of the states agree with experiment to within ~0.5 kcal/mol, achieving “chemical accuracy.” Vibrational energy levels show average deviations of ~20 cm-1 or less. The

$$ B^1 \Delta_g $$

state shows the best agreement with a mean absolute deviation of 2.41 cm-1. Calculated rotational constants exhibit very good agreement with experiment, as do the spectroscopic constants.