 However, in linear molecules many of the electronic states are two-fold degenerate due to "C∞v" or "D∞h" symmetry, and the Born–Oppenheimer approximation breaks down.Since the best-known linear triatomic molecule (CO2) is electronically non-degenerate in its ground state, Renner chose the electronically excited two-fold degenerate Π-state of this well-known molecule as a model for his studies.The products of purely electronic and purely nuclear vibrational states served as the zeroth-order (no vibronic coupling) wave functions in Renner's study.