Revisiting Mulliken's Concepts about Rydberg States and Rydberg-Valence Interactions from Large-Scale Ab Initio Calculations on the Acetylene Molecule

A quantitative characterization of the Rydberg and valence singlet electronic states of acetylene lying in the 5-10.7 eV region is performed by using large-scale ab initio calculations. A special attention is paid on the comparison between the present calculations and Mulliken's concepts for Rydberg states, based on single-electron and single-configuration description. Most of the properties of the Rydberg states have been qualitatively understood via this comparison, mainly shown by the shape and size of the outer Rydberg molecular orbital. More quantitatively, Rydberg-valence mixing has been evaluated in several excited energy regions, as for instance, the interaction between the (C) over tilde' (1 pi(g))(2) (1)A(g) doubly excited valence state and the manifold of electronic components of the np series, or the interaction between the (E) over tilde 1 pi(g) B-1(u) valence state and the (F) over tilde 3d pi(g) (1)Sigma(+)(u) Rydberg state. The rapid predissociation of the lowest (C) over tilde 3s sigma (1)Pi(u) Rydberg state has been interpreted as a case of Rydbergization, earlier predicted by Mulliken.