Vertical spectrum of ethene: uncontracted versus contracted correlation methods and the role of the adapted molecular orbitals

Ethylene vertical excitation energies (VEE) below 9.5 eV have been calculated with the complete active-space singles and doubles configuration interaction self-consistent size-consistent dressing (SC)2CAS–SDCI. The mean of the absolute deviation from experiment for the calculated VEE are 0.14, 0.05 and 0.03 eV for three different molecular orbital (MO) sets tried. The results show that an uncontracted method and a realistic MO set avoid the difficulties present in the calculation of the strongly mixed Rydberg-valence 1B1u states.