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

• Published in: Theoretical chemistry accounts Vol. 133; no. 2
• Main Authors: Boschen, Jeffery S., Theis, Daniel, Ruedenberg, Klaus, Windus, Theresa L.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.02.2014; Springer
• Subjects: Ab initio electronic structure, c2, configuration interaction, configuration-interaction, correlated calculations, COUPLED-CLUSTER, Diatomic carbon, dissociation, gaussian-basis sets, ground-state, infrared bands, molecular electronic wavefunctions, MOLECULE, Multi-configurational wave functions, MULTIREFERENCE, nitrogen molecule, phillips system, spectroscopic properties; Atomic/Molecular Structure and Spectra; Chemistry; Chemistry and Materials Science; Dunning Festschrift Collection; Inorganic Chemistry; MATERIALS SCIENCE; Organic Chemistry; Physical Chemistry; Regular Article; Theoretical and Computational Chemistry; Spectroscopic properties; Multi-configurational wave functions; Diatomic carbon; Dissociation; Configuration interaction; Ab initio electronic structure
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-013-1425-x

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 ( X 1 Σ g + , A 1 Π u , B 1 Δ g , and B ′ 1 Σ g + ) 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 Δ g state as well as an avoided crossing between the two 1 Σ 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 Δ 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.