Importance of multi-reference configuration interaction for 3Σu−←X3Σg− transitions of linear HC7H

Multi-reference configuration interaction (MRD–CI) calculations are required to compute the vertical electronic spectrum of HC7H, because two leading configurations resulting from the excitations 2πu→2πg (HOMO–SOMO) and 2πg→3πu (SOMO–LUMO) are almost equally important in the description of the states 13Σu− and 23Σu−. The electronic absorption spectrum is dominated by the dipole-allowed transitions to these two states from X3Σg−. The lower 13Σu− is computed at 2.55 eV in reasonable agreement with experimental work of 505 nm (2.46 eV), whereas the upper 23Σu− is predicted close to 6.6 eV.