A THEORETICAL DETERMINATION OF THE ELECTRONIC-SPECTRUM OF FORMALDEHYDE

The electronically excited states of formaldehyde are examined by mean s of multiconfigurational second-order perturbation (CASPT2) theory wi th extended ANO-type basis sets. The calculations comprised five valen ce excited states plus all singlet 3s, 3p, and 3d members of the Rydbe rg series converging on the first ionization. The computed vertical ex citation energies were found to be within 0.2 eV of the available expe rimental energies. Full geometry optimization has been performed for f ive valence excited states. Assuming a planar geometry, the ''0-0'' tr ansition for the valence (1)A(1)(pi --> pi) state is calculated to ap pear near 7.9 eV, dose to the (n(y) --> 3p) region. This state is, how ever, not planar and the true adiabatic energy is 7.4 eV, which is 2.3 eV below the corresponding vertical transition.