Theoretical study of C2 and C−2 : X 1Σ+g , a 3Πu , X 2Σ+g , 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 C−2 and the X 1Σ+g and a3Πu 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 3Πu 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 C−2 /C2 is discussed.