Theoretical study of C₂ and C₂^-: X ¹Σ_g⁺, a ³Π_u, X ²Σ_u⁺, and B ²Σ _u⁺ potentials

We employ multiconfigurational self-consistent field and multiconfigurational electron propagator methods to characterize the X ²Σ_g⁺ and B ²Σ _u⁺ states of C₂^- and the X ¹Σ_g⁺ and a ³Π_u, states of C₂ over a wide range of bond lengths (1.0-1.9 Å). We find a systematic difference of approximately 0.3 eV in the relative positioning of our anion- and neutral-state potentials compared to the anion-neutral spacing in the best curves constructed from experimental data. Once this energy shift is taken into consideration, all four of our computed potential energy curves are in reasonably good agreement with experimental information. However, there remains a substantial difference in the relative positioning of our B ²Σ_u⁺ and a ³Π_u curves, compared to the best available experimental data, at larger bond lengths. The relevance of this discrepancy and of our other data to the present state of experimental knowledge on C₂^-/C₂ is discussed.