ELECTRONICALLY EXCITED-STATES OF ETHYLENE

The transition energies for ethylene have been calculated via configuration interaction including all singly excited configurations (CIS) using a variety of basis sets. The minimum requirement for a satisfactory basis set is 6-311(2+)G* having two sets of diffuse functions on the carbon atoms. The excited states were examined via charge density difference plots. The CIS and MP2-corrected CIS (CIS-MP2) methods provided good agreement with experiment for both vertical and adiabatic energies. The equilibrium structures and vibrational frequencies of the excited states and the radical cations were explored, and the CIS, UHF, and UMP2 levels agreed very well with experiment. The nature of the excited states was clearly differentiated by the charge density difference plots and allowed for a reliable and unambiguous assignment of the excited states into both Rydberg and valence classes. Rydberg-valence mixing and the role of these conjugates are also discussed.