Combined SCF and CI Calculations for the Low-Lying Rydberg and Valence Excited States of Ethylene

A series of nonempirical SCF–MO and CI calculations is carried out for the excited states of ethylene. In the usual manner the SCF treatment itself is seen to underestimate vertical transition energies from the closed-shell ground state to open-shell excited states by about 1 eV; an exception is noted, however, in the case of the π → π* singlet–singlet species. A CI(PCMO) treatment, which employs the SCF MO's of a given parent configuration as basis for its own CI expansion, is quite successful in balancing the correlation error, obtaining excellent agreement with experimental transition energies to valence and Rydberg states alike; a possible exception is found in the case of the π → π* singlet–singlet excitation for which the calculated value of 8.32 eV overestimates the location of the V ← N absorption maximum by 0.7 eV. The variational π* MO of the SCF wavefunction for the upper-state singlet is quite diffuse, but it is argued that this fact is not inconsistent with the known experimental data for the V ← N band system. Since the calculated state is found to correlate with a valence species for antiplanar ethylene, its diffuse character in the planar geometry does not imply that its potential surface should resemble that of a Rydberg state; in addition, its charge density contours emphasize that it should not be associated with a pure Rydberg species even in the planar conformation. The change in character with relative rotation of the methylene groups suggests that the electronic transition moment must be considered explicitly in the theoretical treatment of the intensity distribution in the V ← N bands, and also indicates that the probability of nonvertical transitions to partially rotated structures may well be greater than that of the vertical excitation.