CAS calculations of excited electronic states in the vinylidene anion

The CAS calculations with the ANO-L basis were performed for eight electronic states of the vinylidene anion (H2CC−). The results (CASPT2/ANO-L vertical excitation energies and CASSI oscillator strengths) suggest that the observed absorption band of H2CC− at 374 nm is attributed to the X2B2 → 22B2 transition and do not support the previous assignment to the X2B2 → 12A1 transition. The CAS excited-state calculations were also performed for a temporary anion, trans-HCCH−, and the CASPT2/ANO-L results were used for assignment of observed absorption bands of trans-HCCH−. When an ANO-L+ basis (including more diffuse functions) was used, CASSCF wavefunctions for some of the calculated states of the two anions described ‘composed’ systems (neutral species plus free electron) and the results were not reliable.