Vertical ionization potentials of ethylene: the right answer for the right reason?

The photoelectron spectrum of ethylene is studied using coupled cluster methods, including an existing ambiguity in what are reported to be its experimental vertical ionization potentals. Two complementary methods are used for generating the ionization potentials: DeltaE CCSD(T) and IP-EOM-CCSD. The adiabatic IP of the neutral molecule in the ground state is well known and widely accepted to be 10.5122 eV. The basis set extrapolated adiabatic IPs with zero-point corrections are 10.46 eV and 10.56 eV, respectively, but a vibronic coupling between the ground state cation and its first excited state can reduce these values by similar to0.03 eV. From an exponential basis set extrapolation the vertical ionization potentials are predicted to be 10.8 eV (B-3u), 13.2 eV (B-3g), 15.0 eV (A(g)), 16.4 eV (B-2u), and 19.6 eV (B-1u) +/- 0.1 eV.